This chapter was copied with permission from Nick Strobel’s Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version. This chapter has been edited for content (some sections and review questions have been omitted).

Planetary Science

This chapter is an introduction to planetary science. I discuss the techniques astronomers use to find out about the planets, their atmospheres (what determines if an atmosphere sticks around; behavior of gases; what determines the surface temperature; atmosphere layers; the transport of energy; effects of clouds, mountains, and oceans; weather vs. climate and climate change agents with feedbacks; and appearance), their magnetic fields (the magnetic dynamo theory), and their interiors including the geological forces at work reshaping their surfaces. In a separate section I focus on a comparison between the atmospheres of Earth, Venus, and Mars and why they are now so radically different from each other (greenhouse effect, carbon cycle, runaway refrigerator, runaway greenhouse, etc.) Mars discussion now includes proofs for liquid water in past and sub-surface water ice. The Earth discussion now includes the role of plate tectonics in the carbon cycle, evidence for human contribution to the atmospheric carbon dioxide and to the observed global temperature rise. There are links to two flowcharts: a Earth-Venus-Mars comparison and a flowchart of the calculations involved in determining if an atmosphere sticks around for billions of years. I end the chapter with a discussion of the major moons in the solar system and ring systems. Updates include: how paleoclimate is measured, Snowball Earth evidence, Mars discoveries (including notes from January 2011 “Follow the Water” forum), Titan and Enceladus discoveries, and clarifications in discussion of human-caused climate change.

9.1 Introduction

The star we call the Sun has a number of small objects circling around it. Many other stars in our Galaxy have objects orbiting them too and astronomers have recently discovered over four hundred of these other systems already. The largest members of the Sun’s family are called planets, and one of these we call home. That planet, Earth, has many unique characteristics that enable life to exist on it. What are the other planets like? What we call a “planet” has been the subject of much debate recently with the latest definition creating a “dwarf planet” class of which Pluto is a member. A “planet” in our solar system is a celestial body that “(a) orbits the Sun; (b) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and (c) has cleared the neighborhood around its orbit.” See the Kuiper Belt section in the next chapter for more on this.

We have learned more about our solar system in the past few decades than probably any other field of astronomy. The planets are no longer just objects up in our sky, but places we have been to and explored—worlds in their own right. To give an adequate coverage of each of the planets would fill up a whole book (or more)! Since this website is an introduction to all of astronomy, I will not explore each planet individually. Instead, I will focus on the common characteristics of the planets such as their atmospheres, magnetic fields and interiors.

Special attention will be given to the planets Venus and Mars because their orbits are similar to the Earth’s orbit. Venus, Earth, and Mars were made from the same material at about the same distance from the Sun, so they should to be similar to one another. However, they are radically different from one another! Venus is extremely hot with a very thick, carbon dioxide atmosphere, Mars is cold with a very thin carbon dioxide atmosphere, and the Earth has moderate surface temperatures and a moderately-thick nitrogen atmosphere that also has a large amount of very reactive molecules of oxygen. At the end of the chapter I include a section about the large moons of the planets and the rings found around each of the Jovian planets.

The next chapter covers the objects that give us clues of our origins: comets, meteorites, and asteroids. These objects are much smaller than planets and are made of left-over material that did not get incorporated into the planets. I will also give a brief description of what we know about how our solar system was formed in the next chapter.

This chapter covers aspects shared by all of the planets. There is a lot of high-quality information about each of the planets available on the web. Several people and organizations have put together very nice tours of the solar system with actual photos from space missions. Starting points for the best of these tours are given on the Planet Links web page (will display in another window). In addition most of the planet images or their captions are linked to the web sites from where I got the images. Usually, you will be able to find a higher resolution images on those sites. Vocabulary terms are in boldface in this chapter on planetary science.

9.2 Determining Planet Properties

How can you find out what the other planets are like by just observing them carefully from the Earth? Most of the information comes in the form of electromagnetic radiation but we also have little chunks of rock, called meteorites, that give other clues. The image below compares the apparent sizes of the planets. The outer planets are shown at their closest approach to us and the two inner planets are shown at various distances from us (but all are with the same magnification).

Before you can do any sort of comparison of the planets, you need to know how far away they are. Once you know their distances, you can determine basic properties of the planets such as mass, size, and density.

9.2.1 Distances

Several hundred years ago Copernicus was able to determine approximate distances between the planets through trigonometry. The distances were all found relative to the distance between the Earth and the Sun, the astronomical unit.Kepler refined these measurements to take into account the elliptical orbits. However, they did not know how large an astronomical unit was.

To establish an absolute distance scale, the actual distance to one of the planets had to be measured. Distances to Venus and Mars were measured from the parallax effect by observers at different parts of the Earth when the planets were closest to the Earth.

Knowing how far apart the observers were from each other and coordinating the observation times, astronomers could determine the distance to a planet. The slight difference in its position on the sky due to observing the planet from different positions gave the planet’s distance from trigonometry. The state-of-the-art measurements still had a large margin of uncertainty. The last major effort using these techniques was in the 1930’s. Parallax observations of an asteroid, called Eros, passing close to Earth were used to fix the value of the astronomical unit at 150 million kilometers.

With the invention of radar, the distance to Venus could be determined very precisely. By timing how long it takes the radar beam traveling at the speed of light to travel the distance to an object and back, the distance to the object can be found from distance = (speed of light) × (total time)/2. The total time is halved to get just the distance from the Earth to the object. Using trigonometry, astronomers now know that the astronomical unit = 149,597,892 kilometers. This incredible degree of accuracy is possible because the speed of light is known very precisely and very accurate clocks are used. You cannot use radar to determine the distance to the Sun directly because the Sun has no solid surface to reflect the radar efficiently.

9.2.2 Masses

Once you know how far away a planet is, you can use the orbital periods (P) of moons circling a planet and how far the moons are from the planet (d) to measure the planet’s mass. You measure the angular separation between the moon and the planet and use basic trigonometry to convert the angular separation into distance between the planet and moon. That conversion, though, first requires that the distance to the planet and moon be known.

Isaac Newton used his laws of motion and gravity to generalize Kepler’s third law of planet orbits to cover any case where one object orbits another. He found for any two objects orbiting each other, the sum of their masses, planet mass + moon mass = (4π²/G) × [(their distance apart)³/(their orbital period around each other)²]. Newton’s form of Kepler’s third law can, therefore, be used to find the combined mass of the planet and the moon from measurements of the moon’s orbital period and its distance from the planet.

You can usually ignore the mass of the moon compared to the mass of the planet because the moon is so much smaller than the planet, so Kepler’s third law gives you the planet’s mass directly. Examples are given in the Newton’s Law of Gravity chapter. The one noticeable exception is Pluto and its moon, Charon. Charon is massive enough compared to Pluto that its mass cannot be ignored. The two bodies orbit around a common point that is proportionally closer to the more massive Pluto. The common point, called the center of mass, is 7.3 times closer to Pluto, so Pluto is 7.3 times more massive than Charon. Before the discovery of Charon in 1978, estimates for Pluto’s mass ranged from 10% the Earth’s mass to much greater than the Earth’s mass. After Charon’s discovery, astronomers found that Pluto is only 0.216% the Earth’s mass—less massive than the Earth’s Moon! For planets without moons (Mercury and Venus), you can measure their gravitational pull on other nearby planets to derive an approximate mass or, for more accurate results, measure how quickly spacecraft are accelerated when they pass close to the planets.

9.2.3 Sizes and Volumes

The physical size of a planet can be found from measurements of its angular size and its distance. How large something appears to be is its angular size or angular diameter—the angle between two lines of sight along each side of the object. How big something appears to be obviously depends on its distance from us—it appears bigger when it is closer to us. Every time you drive a car or ride a bicycle, you use another car’s or bicycle’s angular size to judge how far away it is from you. You assume that you are not looking at some toy model. The planets are close enough to the Earth that you can see a round disk and, therefore, they have a measurable angular size. All of the stars (except the Sun) are so far away that they appear as mere points in even the largest telescopes, even though they are actually much larger than the planets.

If you know how far away a planet is from you, you can determine its linear diameter D. The diameter of a planet D = 2π × (distance to the planet) × (the planet’s angular size in degrees)/360°, where the symbol p is a number approximately equal to 3.14 (your calculator may say 3.141592653…). The figure above explains where this formula comes from. This technique is used to find the actual diameters of other objects as well, like moons, star clusters, and even entire galaxies.

How do you do that? As the planets orbit the Sun, their distance from us changes. At “opposition” (when they are in the direct opposite direction from the Sun in our sky) a planet gets closest to us. These are the best times to study a planet in detail. The planet Mars reaches opposition every 780 days. Because of their elliptical orbits around the Sun, some oppositions are more favorable than others. Every 15–17 years Mars is at a favorable opposition and approaches within 55 million kilometers to the Earth. At that time its angular size across its equator is 25.5 arc seconds. In degrees this is 25.5 arc seconds × (1 degree/3600 arc seconds) = 0.00708 degrees, cancelling out arc seconds top and bottom.Its actual diameter = (2π × 55,000,000 km × 0.00708°)/360° = 6800 kilometers. Notice that you need to convert arc seconds to degrees to use the angular size formula.

Little Pluto is so small and far away that its angular diameter is very hard to measure. Only a large telescope above the Earth atmosphere (like the Hubble Space Telescope) can resolve its tiny disk. However, the discovery in 1978 of a moon, called Charon, orbiting Pluto gave another way to measure Pluto’s diameter. Every 124 years, the orientation of Charon’s orbit as seen from the Earth is almost edge-on, so you can see it pass in front of Pluto and then behind Pluto. This favorable orientation lasts about 5 years and, fortunately for us, it occurred from 1985 to 1990.

When Pluto and Charon pass in front of each other, the total light from the Pluto-Charon system decreases. The length of time it takes for the eclipse to happen and the speed that Charon orbits Pluto can be used to calculate their linear diameters. Recall that the distance traveled = speed×(time it takes). Pluto’s diameter is only about 2270 kilometers (about 65% the size of our Moon!) and Charon is about 1170 kilometers across. This eclipsing technique is also used to find the diameters of the very far away stars in a later chapter. Pluto’s small size and low mass (see the previous section) have some astronomers calling it an “overgrown comet” instead of a planet and it was recently re-classified as a “dwarf planet.”

Another way to specify a planet’s size is to use how much space it occupies, i.e., its volume. Volume is important because it and the planet’s composition determine how much heat energy a planet retains after its formation billions of years ago. Also, in order to find the important characteristic of density (see the next section), you must know the planet’s volume.

Planets are nearly perfect spheres. Gravity compresses the planets to the most compact shape possible, a sphere, but the rapidly-spinning ones bulge slightly at the equator. This is because the inertia of a planet’s material moves it away from the planet’s rotation axis and this effect is strongest at the equator where the rotation is fastest (Jupiter and Saturn have easily noticeable equatorial bulges). Since planets are nearly perfect spheres, a planet’s volume can be found from volume = (π/6) × diameter³. Notice that the diameter is cubed. Even though Jupiter has “only” 11 times the diameter of the Earth, over 1300 Earths could fit inside Jupiter! On the other end of the scale, little Pluto has a diameter of just a little more than 1/6th the diameter of the Earth, so almost 176 Plutos could fit inside the Earth.

9.2.4 Densities and Compositions

An important property of a planet that tells what a planet is made of is its density. A planet’s density is how much material it has in the space the planet occupies: density = mass/volume. Planets can have a wide range of sizes and masses but planets made of the same material will have the same density regardless of their size and mass. For example, a huge, massive planet can have the same density as a small, low-mass planet if they are made of the same material. I will specify the density relative to the density of pure water because it has an easy density to remember: 1 gram/centimeter³ or 1000 kilograms/meter³.

The four planets closest to the Sun (Mercury, Venus, Earth, Mars) are called the terrestrial planets because they are like the Earth: small rocky worlds with relatively thin atmospheres.

The terrestrial planets to the same scale (using images from NASA and JPL). From top left and proceeding clockwise: Earth, Venus, Mercury, Mars (at bottom left). Terrestrial (Earth-like) planets have overall densities = 4-5 (relative to the density of water) with silicate rocks on the surface. Silicate rock has density = 3 (less than the average density of a terrestrial planet) and iron has a density = 7.8 (more than the average density of a terrestrial planet). Since terrestrial planets have average densities greater than that for the silicate rocks on their surface, they must have denser material under the surface to make the overall average density what it is. Iron and nickel are present in meteorites (chunks of rock left over from the formation of the solar system) and the presence of magnetic fields in some of the terrestrial planets shows that they have cores of iron and nickel. Magnetic fields can be produced by the motion of liquid iron and nickel. Putting these facts together leads to the conclusion that the terrestrial planets are made of silicate rock surrounding a iron-nickel core.

The four giant planets beyond Mars (Jupiter, Saturn, Uranus, Neptune) are called the Jovian planets because they are like Jupiter: large, mostly liquid worlds with thick atmospheres.

The Jovian planets to the same scale (using images from NASA and JPL). From top left and proceeding clockwise: Jupiter, Uranus, Neptune, Saturn (at bottom right). The Earth is also included to the same scale at center.

Jovian (Jupiter-like) planets have overall densities = 0.7-1.7 (relative to the density of water) with light gases visible on top. Gases and light liquids (like hydrogen and helium) have densities lower than water. Using reasoning similar to before you conclude that the Jovian planets are made of gaseous and liquid hydrogen, helium and water surrounding a possible relatively small rocky core. Spectroscopy says the Jovian planets have hydrogen, helium, methane, ammonia, and water gas in their thick atmospheres so the predictions are not too far off track.

The properties determined for each planet are given in the Planet Properties table. Clicking on the planet’s name will bring up the full fact sheet for that planet. The important properties are given in the table.

Vocabulary

• angular diameter
• angular size
• center of mass
• density

Formulae

1. Planet mass = (4π²/G) × [distance³/(moon’s orbital period)²] – moon’s mass. The moon’s mass can usually be ignored.
2. Planet diameter = 2π × (distance to the planet) × (planet’s angular size in degrees)/360°.
3. Planet volume = (π/6) × (planet diameter)³.
4. Density = mass/volume.

Review Questions 1

1. How does angular size depend on distance? (As its distance increases, does an object’s angular size increase or decrease?)
2. How large is the Moon if its angular size is about 30 arc minutes and it is 384,000 kilometers away from us?
3. How many times bigger is the Sun than the Moon if the Sun is about 390 times farther away than the Moon but has the same angular size? What is the Sun’s diameter in kilometers? (Hint: use the Moon data given in the previous question.)
4. If two objects at the same distance from us have different angular sizes with one 22 times smaller than the other, how many times larger in linear diameter is one than the other? If the large object is 141,000 kilometers across, how large is the other object?
5. A planet 140,000 kilometers across is 780 million kilometers from the Sun and rotates every 9.8 hours on its axis. It has a moon that orbits 380,000 km from the planet’s center in 1.8 days. Explain what information you would use to find the mass of the planet and how the mass could be determined.
6. The Earth has a radius of 6400 kilometers and a mass of 6.0 × 10²⁴ kilograms. What is its density?
7. A planet has a rocky surface of material with density 3 (relative to the density of water) and an interior of material with density 2.5. Will its overall average density be greater or less than 3? If the planet’s interior is made of material with density 9, will the overall average density be greater or less than 3?

9.3 Atmospheres

A planet’s atmosphere helps shield a planet’s surface from harsh radiation from the Sun and it moderates the amount of energy lost to space from the planet’s interior. An atmosphere also makes it possible for liquid to exist on a planet’s surface by supplying the pressure needed to keep the liquid from boiling away to space—life on the surface of a planet or moon requires an atmosphere. All of the planets started out with atmospheres of hydrogen and helium. The inner four planets (Mercury, Venus, Earth, and Mars) lost their original atmospheres. The atmospheres they have now are from gases released from their interiors, but Mercury and Mars have even lost most of their secondary atmospheres. The outer four planets (Jupiter, Saturn, Uranus, and Neptune) were able to keep their original atmospheres. They have very thick atmospheres with proportionally small solid cores while the the inner four planets have thin atmospheres with proportionally large solid parts.

The properties of each planet’s atmosphere are summarized in the Planet Atmospheres table (will appear in a new window). Two key determinants in how thick a planet’s atmosphere will be are the planet’s escape velocity and the temperature of the atmosphere.

9.3.1 Escape of an Atmosphere

The thickness of a planet’s atmosphere depends on the planet’s gravity and the temperature of the atmosphere. A planet with weaker gravity does not have as strong a hold on the molecules that make up its atmosphere as a planet with stronger gravity. The gas molecules will be more likely to escape the planet’s gravity. If the atmosphere is cool enough, then the gas molecules will not be moving fast enough to escape the planet’s gravity. But how strong is “strong enough” and how cool is “cool enough” to hold onto an atmosphere? To answer that you need to consider a planet’s escape velocity and how the molecule speeds depend on the temperature.

9.3.1.1 Escape Velocity

If you throw a rock up, it will rise up and then fall back down because of gravity. If you throw it up with a faster speed, it will rise higher before gravity brings it back down. If you throw it up fast enough it just escapes the gravity of the planet—the rock initially had a velocity equal to the escape velocity. The escape velocity is the initial velocity needed to escape a massive body’s gravitational influence. In the Newton’s Law of Gravity chapter the escape velocity is found to = Sqrt[(2G × (planet or moon mass))/distance)]. The distance is measured from the planet or moon’s center.

Since the mass is in the top of the fraction, the escape velocity increases as the mass increases. A more massive planet will have stronger gravity and, therefore, a higher escape velocity. Also, because the distance is in the bottom of the fraction, the escape velocity decreases as the distance increases. The escape velocity is lower at greater heights above the planet’s surface. The planet’s gravity has a weaker hold on the molecules at the top of the atmosphere than those close to the surface, so those high up molecules will be the first to “evaporate away.”

Do not confuse the distance from the planet’s center with the planet’s distance from the Sun. The escape velocity does NOT depend on how far the planet is from the Sun. You would use the Sun’s distance only if you wanted to calculate the escape velocity from the Sun. In the same way, a moon’s escape velocity does NOT depend on how far it is from the planet it orbits.

9.3.1.2 Temperature

The temperature of a material is a measure of the average kinetic (motion) energy of the molecules in the material. As the temperature increases, a solid turns into a gas when the particles are moving fast enough to break free of the chemical bonds that held them together.

The particles in a hotter gas are moving quicker than those in a cooler gas of the same type. Using Newton’s laws of motion, the relation between the speeds of the molecules and their temperature is found to be temperature = (gas molecule mass)×(average gas molecule speed)² / (3k), where k is a universal constant of nature called the “Boltzmann constant.” Gas molecules of the same type and at the same temperature will have a spread of speeds—some moving quickly, some moving slower—so use the average speed.

If you switch the temperature and velocity, you can derive the average gas molecule velocity = Sqrt[(3k × temperature/(molecule mass))]. Remember that the mass here is the tiny mass of the gas particle, not the planet’s mass. Since the mass is in the bottom of the fraction, the more massive gas molecules will move slower on average than the lighter gas molecules. For example, carbon dioxide molecules move slower on average than hydrogen molecules at the same temperature. Because massive gas molecules move slower, planets with weaker gravity (e.g., the terrestrial planets) will tend to have atmospheres made of just massive molecules. The lighter molecules like hydrogen and helium will have escaped.

The dependence of the average speed of the gas molecules on their mass also explains the compositional structure observed in planet atmospheres. Since the distance a gas molecule can move away from the surface of a planet depends only on how fast it is moving and the planet’s gravity, the lighter gas molecules can be found both close to the surface and far above it where the gravity is weaker. The gas molecules high up in the atmosphere are most likely to escape. The massive gas molecules will stay close to the planet surface. For example, the Earth’s atmosphere is made of nitrogen, oxygen, and water molecules and argon atoms near the surface but at the upper-most heights, hydrogen and helium predominate.

Whereas the process described above leads to evaporation molecule by molecule, another type of atmospheric loss from heating happens when the atmosphere absorbs ultraviolet light, warms up and expands upward leading to a planetary wind flowing outward to space. Planets with a lot of hydrogen in their atmospheres are especially subject to this sort of atmospheric loss from heating. The very light hydrogen can bump heavier molecules and atoms outward in the planetary wind.

9.3.1.3 Does Gravity Win or Temperature?

The effects of gravity and temperature work opposite to each other. A higher temperature tries to dissipate an atmosphere while higher gravity tries to retain an atmosphere. If the particle’s average speed is close to the escape velocity, then those type of gas particles will not remain for billions of years. The general rule is: if the average gas molecule speed for a type of gas is less than than 0.2×(the escape velocity), then more than 1/2 of that type of gas will be left after one billion years. If the average speed is greater than that critical value, then more than 1/2 of that type of gas will be gone after one billion years. A flowchart of this is given on the escaping atmosphere page.

Because the Jovian planets are massive and cold, they have THICK atmospheres of hydrogen and helium. The terrestrial planets are small in mass and warm, so they have thin atmospheres made of heavier molecules like carbon dioxide or nitrogen.

Test and improve your understanding of these concepts with the UNL Astronomy Education program’s Atmospheric Retention module (link will appear in a new window). Note that it does use some simplifications but it provides a nice way to show the roles of temperature and escape velocity in determining how thick a planet’s atmosphere will be.

9.3.1.4 Atmosphere Escape via Non-Thermal Processes

The processes described above occur from the heating of the atoms and molecules in an atmosphere to the point where they can escape the planet’s gravity. They are called thermal processes. Other ways involve the presence or lack of a magnetic field and asteroid or comet impacts. Ions are atoms that have an extra charge (usually by losing an electron). Ions will spiral around magnetic field lines so a planet’s magnetic field (discussed more in a later section) will have a lot of ions trapped in it. When a fast-moving hydrogen ion (a proton) bumps into a neutral atom it can steal an electron to become a neutral atom that is not trapped by the magnetic field and it escapes the planet’s gravity. This is called charge-exchange. Some of the magnetic field lines are so wide that they get stretched out by the high-speed stream of ions from the Sun called solar wind. The stretched out lines do not loop back and just open out into interplanetary space. Ions spiraling around these open magnetic field lines can escape along those lines in what is called a polar wind.

If a planet does not have a magnetic field (for reasons described later), the solar wind can strip an atmosphere through a process called sputtering. Without a magnetic field, the solar wind is able to hit the planet’s atmosphere directly. The high-energy solar wind ions can accelerate atmosphere particles at high altitudes to great enough speeds to escape. An additional way of atmosphere escape called photodissociation occurs when high-energy sunlight (e.g., ultraviolet or x-rays) hits high-altitude molecules in the planet’s atmosphere and breaks them apart into individual atoms or smaller molecules. These smaller particles have the same temperature as the larger molecules and, therefore, as described above, will move at faster speeds, possibly fast enough to escape.

The processes described so far in this section work particle to particle and work over long time periods as the atmosphere leaks away particle by particle. In contrast impacts by comets or asteroids can inject a huge amount of energy very quickly when the projectile vaporizes upon impact. The expanding plume of hot gas drives off the air above the impact site, with the larger the impact energy, the wider is the cone of air that is removed above the impact site. The impact removal process was probably particularly effective for Mars (being so close to the asteroid belt) and the large moons of Jupiter (so close to Jupiter’s strong gravity that attracts numerous comets and asteroids).

9.3.2 Gas Behavior

Now that we have seen what determines if a planet has an atmosphere or not, let’s look at how atmospheres behave and how they affect the conditions on the surface of a planet. Only three things are needed to describe how a gas will behave: temperature, pressure, and density. Temperature and density have already been discussed, although for gases it is usually easier to use “number density” as in “number of particles per volume” instead of the usual mass density. Let’s take a closer look at pressure.

9.3.2.1 Pressure

Pressure is the amount of force exerted on a surface per unit area such as a number of newtons per square meter (or pounds of force per square inch). In the metric system, the unit of pressure is the pascal (Pa). This pressure is supplied by all of the particles in an atmosphere colliding into each other. At the Earth’s surface there are approximately 25 million trillion (2.5 x 10¹⁹) molecules in every cubic centimeter moving about at speeds of hundreds of meters per second, so yes, they are going to bump into each other!

At the Earth’s surface at sea level there is about 100 kilopascals of pressure exerted all over your body. For planet atmospheres, scientists will usually use a unit called a “bar” equal to 100 kilopascals, so the Earth’s surface pressure at sea level is about 1 bar. That pressure is equivalent to about 1 kilogram pressing down in one Earth gravity on every square centimeter. If an adult pinky is about 1 centimeter in width (more or less), your body has quite a lot of square centimeters of surface area. So why do you not feel all that weight pushing you down at the Earth’s surface? As described in the figure below, the air pressure pushes in all directions equally and the pressure of the fluid in your body pushing outward balances the air pressure.

9.3.2.2 Ideal Gas Law

Given the three parameters of temperature, density, and pressure, how the gas behaves is described by the equation of state. Most gases will obey a simple equation of state called the ideal gas law in which a doubling of the temperature or a doubling of the (number) density leads to a doubling of the pressure. For example, if you blow twice as much air into a balloon, the gas inside the balloon will push outward with twice as much pressure and the elastic material will expand until a new pressure balance is reached with the outside air pressure. Heating the air inside a hot air balloon increases the pressure inside the fabric enclosure so the balloon fabric that started out laid out all flat on the ground is now puffed into a round shape. (This also explains why your car’s manual will tell you to measure the air pressure of your tires when they are cold, so you if you have been driving for a while, you will need to wait several minutes at least for the air inside the tire to cool off to get an accurate tire pressure reading.)

Let’s continue with the hot air balloon example to make another important point. Once the balloon fabric is all puffed up, raising the temperature further inside the balloon will cause the air inside to flow out of the hole in the bottom of the balloon and density inside will drop. At the same pressure, less dense things will float upward (Archimedes’ principle)—the hot air balloon will rise up off the ground. At a given pressure, cooler air is more dense than hotter air so the cooler air will sink. In an atmosphere, rising warmer air and sinking cooler air can transfer heat energy from a hotter surface to a cooler upper layer of the atmosphere in a process called convection that will be covered in more detail later.

9.3.3 Hydrostatic Equilibrium

Gravity pulls downward/inward on the molecules in an atmosphere but atmospheres remain “puffy” because of the moving gas particles supply pressure upward/outward. An atmosphere will not get puffier or shrink if the outward thermal pressure of the gases is balances by the inward gravity compression. This balance between pressure and gravity is called hydrostatic equilibrium. In the interior of planets, the resistance of the solid or liquid material supplies the pressure. In an atmosphere, the moving gases supply the pressure. If the Earth was at the distance of Pluto from the Sun, the nitrogen, oxygen, water, etc. in our air would freeze out and gravity would cause it all to collect on the surface about 12 meters thick. At our warmer position, these materials are in a gaseous state and make a layer 100 kilometers thick. [More accurately, our atmosphere extends out even further, beyond where the Space Shuttle can reach and where the International Space Station is but the air pressure is extremely small so 100 kilometers has been set as the “boundary” (fuzzy though it is) where space begins. Objects in low Earth orbit do feel a slight drag though and therefore need to be periodically boosted back up to their original orbit or they will spiral downward and burn up in our atmosphere.]

Lower layers of the atmosphere feel greater gravity compression from all of the material in the layers above pushing down on them. Therefore, they exert greater pressure to keep the balance.

9.3.4 Surface Temperature

What temperature is and how it relates to a particle’s speed and mass and how a gas’ temperature affects the pressure has been discussed above but what will raise or lower the temperature of the air, particularly the air at the surface of a terrestrial world or the upper layers of a Jovian planet’s atmosphere? The surface temperature of a terrestrial planet is determined by how much energy the planet receives from the Sun and how quickly it radiates that solar energy back to space. As described in a later section, a terrestrial planet’s interior temperature is determined by its size. The crust is a very poor conductor of any heat from the interior so the surface heat is all from the Sun. Jupiter, Saturn, and Neptune have extra heat energy coming from their interiors. Recall from the section about seasons that in order to keep the temperature the same, there must be a balance between the solar energy flowing onto the planet and the energy radiated back out to space.

On global scales, three things can affect this energy flow and therefore, the average global surface temperature. As shown in the figure below, they are the planet’s distance from the Sun, the planet’s surface reflectivity (albedo), and the planet’s atmosphere (through a process called the greenhouse effect).

Planets closer to the Sun receive more solar energy by an amount that depends on their distance squared (recall how light spread out as described in the telescope chapter). Also recall from the light chapter that hotter dense things produce more energy (they are brighter). With more solar energy flowing to the closer planets, they must be hotter to re-radiate that energy back out to space. The amount of solar energy reflected immediately out to space is determined by the material on the planet’s surface or clouds in the atmosphere. The fraction of sunlight that is reflected from an object is the albedo. If the albedo is closer to 1 (100% reflectivity), the planet does not need to be as hot to have its outflow of energy balance the inflow of solar energy. Darker objects absorb more solar energy and, therefore, they need to heat up more to re-radiate that energy back out to balance the inflow of solar energy. For example, you probably notice the difference between wearing a white T-shirt vs. wearing a black T-shirt outside on a sunny, summer day. Another example is the liquid water in our oceans absorb more solar energy than the ice areas at our poles.

Let’s pause here and find out what the surface temperature should be for some of the planets. The rate of energy absorbed by the planet equals (the absorbing area of the planet) x (brightness of sunlight at the planet’s distance from the Sun) x (fraction of sunlight absorbed). The rate of energy radiated by the planet using the discussion in the light chapter (see item 5 in the list on that webpage) equals (the surface area of the planet) x (energy radiated by each square meter every second, which changes as temperature to the fourth power). Setting the rate of solar energy absorbed equal to the rate of energy radiated by the planet, you find using the values for the distances and albedo given in the planet tables appendix, that Mercury should have an average temperature of 160 deg C, Venus should be -42 deg C, Earth should be -19 deg C, and Mars should be -63 deg C. Their actual temperatures are: Mercury = 425 deg C (day)/ -175 deg C (night) = midway value of 125 deg C; Venus = 464 deg C; Earth = 15 deg C; and Mars = -31 deg C (day)/-89 deg C (night) = midway value of -60 deg C. Mercury has such a large variation between day and night as well as a large variation between the daytime polar temperature of 317 deg C and daytime equatorial temperature of 452 deg C that the rather arbitrarily chosen midway value of 125 deg is close enough to the calculated value of 160 deg C.

How do you do that? To find the expected temperature of a planet, you need to set the rate of energy radiated by the planet equal to the rate of energy absorbed by the planet. Using the terms in the main text above, you find , where R is the radius of the planet, d is the distance from the Sun, a is the albedo, Lsun is the luminosity of the Sun (how much energy it emits every second), T is the temperature of the planet, and σ is the Stefan-Boltzmann constant (see the astronomical constants table in the appendix for its value). Using algebra to solve for just the temperature: . Plugging in the values for the constants from the astronomical constants table in the appendix, you find Kelvin if the distance is measured in astronomical units. Finally, to convert to degrees Celsius, subtract 273 from the Kelvin value. For example, for the Earth, the albedo is 0.306 and the distance is 1.000 AU, so the expected temperature is 254 K or -19 C —significantly below the freezing point of water!

The difference between actual temperature values and the calculated values for the other three planets are a bit more interesting because of the effect of their atmospheres. A planet’s atmosphere can hinder the rate that energy flows outward to space from the warm ground so the ground must heat up to increase the energy leaking out enough to balance the inflow of solar energy. This blanket effect of the atmosphere is called the greenhouse effect.

9.3.4.1 Greenhouse Effect

The greenhouse effect is named after the glass houses used to keep plants warm during cold weather. Energy in the form of visible light from the Sun passes through the glass walls and glass roofs of a greenhouse and heats up the plants and soil inside the greenhouse. The air in contact with the plants and soil gets warmed up. The glass walls and roofs prevent the hot air from escaping to the outside. The same sort of thing happens to the interior of your car when you leave it out in the Sun with the windows rolled up.

On a planet, certain gases like carbon dioxide or water vapor in the atmosphere prevent heat energy in the form of infrared light from leaking out to space. These so-called “greenhouse gases” allow visible light from the Sun to pass through and heat up the surface. A planet’s surface is warm enough to emit infrared light. Some of the infrared light is absorbed by the greenhouse gases and radiated back toward the surface, warming the surface even more. Some of the energy is radiated back toward space. The surface warms enough so that the amount that does leak back out to space balances the solar energy flow inward. Note that if the greenhouse was a perfect blanket, then the surface would continue to get hotter and hotter.

The primary greenhouse gases found in the atmospheres of our solar system’s planets are given in the figure below. Not shown are chlorofluorocarbons (CFC’s) and hydrofluorocarbons (HFC’s) that are synthesized by humans. On the Earth the relative amounts these molecules contribute to the total greenhouse effect occurring are approximately: 60% for water, 26% for carbon dioxide, 5% for methane, 4% for ozone, 4% for the CFC’s/HFC’s, and 2% for nitrous oxide (rounding of the numbers to integer values means they will not add up to exactly 100%).

Now back to our three planets. Venus’ actual temperature is over three times more than if there was no greenhouse effect at work. Earth has a natural greenhouse effect mostly caused by water vapor to raise the temperature by about 34 deg C so the oceans do not freeze. Mars has only a very slight warming because of its thin atmosphere.

9.3.5 Atmosphere Structure

Planet atmospheres have a layered structure based on how the temperature changes with increasing altitude. The greenhouse effect plays a major role for the lowest layer of a terrestrial planet’s atmosphere and other heating agents can raise the temperature of the upper layers. In this section we will take a look at the Earth’s atmosphere layers, then compare it to the other terrestrial planet atmospheres, and finally finish with the structures of the Jovian planets atmospheres.

The figure above shows the bottom four layers of the Earth’s atmosphere. (Standard model atmosphere from Steven Pietrobon at Small World Communications.) Here are short descriptions of each layer.

• Troposphere: lowest layer (closest to the ground). The greenhouse effect is present in some amount. The temperature drops with increasing altitude because of more greenhouse heating lower down. Convection is important. In fact, without convection the temperature difference between the mountain tops and sea level would be even greater. The churning of the air by convection makes our storms. Ultimately, solar energy is what powers our storms. Clouds of water droplets and ice crystals are found in here. Other planets will have clouds made of other molecules in here.
• Stratosphere: where the temperature begins rising with increasing altitude above the troposphere. Ultraviolet light is absorbed by the ozone molecules in this layer. Ozone is the molecule made up of three oxygen atoms you came across in the greenhouse section above. It is beneficial to life when it is up in the stratosphere. Upon absorbing the ultraviolet light the fragile ozone molecules break apart. They re-form later when an oxygen atom combines with an oxygen molecule to complete the cycle. The absorption of the ultraviolet light is why the temperature increases. Ozone in the stratosphere is considered “good ozone” because of its shielding effect. Ozone in the troposphere is considered “bad ozone” because it causes respiratory problems and other negative health effects as well as being destructive to organic materials such as plastics.
• Mesosphere: where the temperature begins falling again with increasing altitude above the stratosphere because there is no ozone to absorb the ultraviolet light.
• Thermosphere: where the temperature begins rising again with increasing altitude as the gases absorb X-rays and some ultraviolet light and heat up. No X-rays reach below the thermosphere. The X-rays have enough energy to knock electrons out of atoms making the atoms charged, a process called ionization. Where the ionization happens the most is called the ionosphere, a layer important for radio communication because the radio waves reflect off this layer and enable them to travel beyond the line-of-sight horizon. Aurorae occur in this layer (described in a later section).
• Exosphere: the uppermost layer where the gases escape to space at about 500 kilometers or so from the Earth’s surface. Very low density gases heated by X-rays and ultraviolet light. Mercury and the Moon technically also have exospheres but their exospheres begin right at their surfaces.

Both Mars and Venus have tropospheres of greater extent than the Earth, though for different reasons. Mars’ atmosphere is much thinner than Earth’s and there is less compression because of Mars’ weaker gravity. Although Venus has weaker gravity than Earth, it has over ninety times the amount of atmosphere because of a runaway greenhouse effect that occurred at least hundreds of millions of years ago. That will be described further later but it does provide a warning to us that drastic global climate change is possible. Because their tropospheres extend over a greater distance than the Earth’s troposphere, Mars’ and Venus’ clouds are found at higher altitudes. Mars and Venus also have thermospheres. What is missing is the temperature bump of a stratosphere (and mesophere) because they do not have an ozone layer to absorb the ultraviolet light. (Planet atmosphere models courtesy of Jere Justus at the Marshall Space Flight Center, NASA.)

The Jovian planets have the same atmosphere layers as the Earth, though, their compositions, of course, are very different. Jupiter’s atmosphere structure is described in the figure below.

Jupiter’s troposphere extends much further down merging smoothly into its interior. Mixed in with the abundant molecular hydrogen and helium are trace amounts of ammonia, water, and methane. Even smaller amounts of hydrogen sulfide (the stinky stuff of rotten eggs), other hydrogen polysulfides, and phosphorus are also present. Ammonia and hydrogen sulfide will mix together in water to make ammonium sulfide. Molecules of ammonia, ammonium hydrosulfide, and water will form droplets (condense) when the temperature is low enough. They will condense (and freeze) at different temperatures, though, so clouds of these molecules will form at different depths in the troposphere. There are three main clouds decks on Jupiter. Water condenses at a higher temperature than the other two, so water clouds are thought to exist at the deepest cloud layer. Higher up the temperature is low enough for the ammonium hydrosulfide to condense. Finally, just below the upper bound of the troposphere, the temperature is cold enough for ammonia to condense. Note that the cloud layers mark the upper bound of that type of molecule in the hydrogen/helium atmosphere.

The Galileo spacecraft dropped a probe into Jupiter’s atmosphere when it arrived at Jupiter in December 1995. The probe got a depth of 161 km below the cloudtops before the probe stopped functioning because of high pressures and temperatures. At that depth the pressure was 22 bars and the temperature about 425 K. From the atmosphere structure figure above, the Galileo probe should have penetrated to where the water vapor is but it did not find the water. Unfortunately, the probe entered one of the clear, dry areas produced by downdrafts. Although, the probe got over 160 km below the cloudtops, it was still in the troposphere and its deepest point represents just 0.3% of Jupiter’s radius. Above the troposphere of Jupiter, ultraviolet heating makes a stratosphere but other molecules instead of ozone absorb the ultraviolet light. At the highest levels are the thermosphere and the exosphere.

Saturn has the same three main cloud decks, though they are found at lower altitudes than on Jupiter because of Saturn’s lower temperature (it is further from the Sun). Also, Saturn’s cloud decks are further apart from each other because of Saturn’s lower gravity—there is less compression of the gases. Because Saturn’s clouds form at deeper positions in its troposphere, its cloud patterns appear more muted than on Jupiter. It is thought that the three cloud decks (of ammonia, ammonium hydrosulfide, and water) would be found much too deep in the tropospheres of Uranus and Neptune for us to see. Instead, Uranus and Neptune being even further from the Sun have cold enough tropospheres for methane to condense and freeze to form clouds (Jupiter and Saturn are too warm for methane clouds). Neptune has extra heat energy from its interior so its atmosphere temperature is warmer than Uranus.

In the next section we will take a look at how the gases move about and distribute energy.

9.3.6 Energy Transport

Planet atmospheres slow down the rate that heat escapes from a planet’s surface or interior, as well as, distributing heat over a planet’s surface. Because there is a temperature difference between the surface or interior of a planet and space, heat energy will flow from the warmer surface to the cold space. How well an atmosphere transfers heat and the methods it uses will have a profound effect on the surface temperature and weather. There are three ways gases can transfer energy:

1. radiation—photons (energy packets) leak outward by scattering off gas particles. Nature prefers this way.
2. conduction—fast-moving atoms collide with other atoms imparting some of their motion to them. This is used by metals like copper or aluminum to transfer heat (eg., from your stove element to the food), but it is not used by a gas since the gas molecules are so far apart from one another. The process of conduction is too inefficient in a gas to worry about. (This is why you can stick your hand into your oven while something is baking and not immediately burn your hand if it does not touch anything, especially the metal sides and rack.)
3. convection—big pieces of the atmosphere cycle between cold regions and warm regions. Hot air below expands and its density decreases so it rises. Cooler, denser air falls and displaces the hot air. As a hot bubble rises, it cools by giving up its heat energy to the cool surroundings. The gas will then fall and heat up when it comes into contact with the warm surface or interior.
4. It is a sort of “energy conveyor belt” motion of gas. Nature will use convection only if there is a large change in temperature over a small distance (a “steep temperature gradient”). Such conditions are found in planet atmospheres (compared to size of a planet, atmospheres are very thin!) and the interiors of stars.

In addition to transporting energy outward to space, convection also distributes the heat across the planet, from the warm daylit equatorial regions to the cooler latitudes closer to the poles and to the night side of the planet. The warm air at the equatorial regions rises and the cooler air from other parts of the planet flows across the surface toward the equator to replace the rising air. All of the winds in a planet’s atmosphere are due to convective processes. If the planet is rotating quickly enough, the motion of the air can be deflected sideways by the Coriolis effect (see also the Galileo section in the history chapter).

 

If a pocket of air from the pole moves toward the equator without changing direction, the Earth will rotate beneath it. The packet of air has a sideways motion equal to the rotation speed at the pole, but the parts of the Earth’s surface closer to the equator have a greater rotational speed because they are farther from the rotation axis. To an observer on the ground, the path appears deflected to the west. The Coriolis deflections produce the spiral patterns of cyclonic storms and air flow away from high-pressure regions.

 

Some nice animations of air circulation around low and high pressure regions are available from NASA’s Visible Earth site: low-pressure circulation animation — high-pressure circulation animation.

The rapid rotation of a planet will also complicate the convective flow of energy from the warm equator to the cool poles. On a planet with little or no rotation (Venus, for example), the air circulation is very simple: warm air rises along the equator, flows at high altitudes toward the poles, and near the surface returns to the equator. On a planet with rapid rotation (Earth or the jovian planets, for example), the surface winds from the poles are deflected into large-scale eddies with belts of wind and calm. At high altitudes narrow bands of high-speed winds called jet streams are formed and they play an important role on the surface weather. Land masses sticking up into the air flow disrupt the spiralling circulation and provide a place for storms to expend their energy.

 

The rapidly rotating jovian planets have much greater Coriolis effects. The powerful, narrow jet streams deflect the clouds into belts moving parallel to the planet equators. The winds in a belt move in the opposite direction of the belt next to it. Large vortices can form from the interplay of the belts. A large vortex can last for decades, even centuries or longer because the jovian planets do not have a solid surface for storms to expend their energy. Jupiter’s Great Red Spot is an example of a large vortex. Twice the size of the Earth, it is at least 400 years old.

Image from the Voyager spacecraft courtesy of NASA

For most of the planets, the Sun provides the energy to maintain the temperature (and surface temperature for the terrestrial planets) and to drive the convective motions of the atmosphere. But Jupiter, Saturn, and Neptune generate over twice as much heat than they receive from the Sun. Most of this energy is left over heat from when the planets formed 4.6 billion years ago. As material collected onto the forming planets, it heated up when energy was released by the material falling in the planet’s gravity field. All of the planets were hot enough to be liquid. The heavier, denser materials (like iron and nickel) separated from the lighter materials (like silicon, hydrogen, and helium) and fell toward the planet cores. The process called differentiation released more gravitational energy and heated up the planets further. Due to their large size, the jovian planets still retain a lot of their initial formation heat and that energy is responsible for the spectacular clouds patterns. In the case of Saturn, the differentiation process may still be going on as the helium in the interior separates from the hydrogen and sinks toward the core, a “helium rain”. The helium rain is probably why there is a smaller percentage of helium in Saturn’s atmosphere than in Jupiter’s atmosphere.

The much blander atmosphere of Uranus is a result of its lower heat emission. Most of the heat inside the much smaller Earth and Venus is produced from radioactivity in the rocky material (in fact, the higher radioactive heating long ago may have been necessary for the terrestial planets to undergo differentiation). However, the heat of Venus’ and Earth’s interior has little to zero effect on their atmospheres because the crust is such a poor conductor of heat (though convection in their interiors is responsible for the geologic processes seen on their surfaces). Sunlight energy is what determines their surface temperatures and drives their weather.

Atmospheres moderate the heat lost to space at night and shield the planet surface from energetic radiation like solar ultraviolet and X-rays and the high-speed charged particles in solar wind and mostcosmic rays (extremely high-energy particles from space, mostly protons). The planet Mercury has almost no atmosphere and so there is a difference of several hundred degrees between places in the shade and sunlit areas! The planet Mars has a very thin atmosphere, so it experiences a temperature drop of over a 100 degrees when night comes. Humans landing on the martian surface will need to contend with the extreme cold of the night and will need to protect themselves from the harmful solar radiation during the day. The Earth’s atmosphere is thick enough that the temperature difference between night and day is at most a few tens of degrees. Our atmosphere also blocks high-energy light like UV and X-rays and solar wind particles. Some cosmic ray particles have high enough energy to penetrate the atmosphere and even several meters of rock! If a cosmic ray strikes the DNA in the cells, the DNA structure can be altered. Cosmic rays are responsible for some of the genetic mutations in life.

9.3.7 Clouds + Air Circulation

Clouds form when the temperature is cool enough for certain molecules in a planet’s atmosphere to form droplets (condense). Clouds are usually made from minor ingredients in a planet’s atmosphere (for example, Earth’s clouds are made of water droplets that makes up less than 1% of our air globally while the majority of Earth’s atmosphere is nitrogen and oxygen).

Clouds are important because (1) they block input energy from the Sun (a cooling effect); and (2) they trap energy from the surface (a heating effect). It is difficult to figure out which effect has greater long-term significance in how the temperature will change over time periods of years. Clouds are also extremely hard to model because they are so variable and changeable. The formation of clouds is also part of an energy transfer process because of an energy called latent heat. When a liquid turns into a gas such as liquid water evaporating, it absorbs energy while the temperature remains constant. The absorbed energy is the latent heat. (This is why we sweat—as the droplets of sweat produced by our bodies evaporate, they absorb energy from our overheated body taking away the excess energy we produce during exercise.) That latent heat is released when the gas condenses into droplets (clouds form). On the Earth, water evaporation and condensation provide a major source of energy transfer to drive the winds and they are key parts of the water cycle (hydrological cycle) shown in the figure below.

Water from the oceans, rivers, and lakes evaporates to become water vapor. Warm air is able to hold more water vapor than cool air so as convection in the troposphere moves air upward, the water vapor will condense at the cooler altitudes. The cloud droplets (or crystals) will grow as they pick up more water vapor. Eventually, the cloud droplets grow so large that the cooler air cannot hold them anymore and they fall to the surface. The rainwater (or snowmelt during warm days) runs downhill starting out as streams that flow into small rivers that in turn flow into large rivers down to the oceans. Water also percolates into the soil to become part of the groundwater that totals approximately 10% of the mass of the oceans. Some of the groundwater collects in reservoirs underground called aquifers. Groundwater can remain in aquifers for a million years or more. Some aquifer water can reach the surface and flow out to the oceans. Water that falls as snow can also store water for long periods of time as ice sheets and glaciers near the poles or a high elevations.

A similar sort of evaporation-condensation-precipitation cycle is found on other planets but the molecule may be something other than water. The situation closest to what happens on the Earth is what is found on Titan, a moon of Saturn. There, methane is the key player, instead of water. In fact, Titan is so cold that water is in deep freeze, so water there plays the role of rock here.

9.3.7.1 Mountains

The figure also shows the effect of mountains. The mountains on the Earth are high in comparison to the thickness of our troposphere so they can provide a significant barrier to the circulation of air or change the circulation patterns. They can also “speed up” the condensation-precipitation process by forcing side-ways moving air to rise upward leading to storms on the mountainsides that face the prevailing winds. Air on the other side of the mountains is much drier so the land on the other side has little rainfall—a “rain shadow“. Some example rain shadows include the southern part of the San Joaquin valley in California (formed by the Diablo Range and the Tehachapi Mountains), the eastern half of Oregon and Washington (formed by the Cascade Range), and the Atacama Desert (the driest place on Earth formed by the Chilean Coast Range).

9.3.7.2 Ocean Currents

Our oceans also affect atmospheric circulation by transporting heat energy and water, heating or cooling the land and air. For example, the California Current is a current in the Pacific Ocean that moves southward along the western coast of North America, from British Columbia to southern Baja California. The movement of the cooler northern waters southward makes the coastal waters cooler than the coastal areas of the eastern United States at the same latitudes. The Gulf Stream is a warm surface ocean current born in the Gulf of Mexico that flows up the eastern United States seaboard and then veers northeastward toward Europe at about 6 km/hour (4 mph). The North Atlantic Drift (aka North Atlantic Current) continues carrying the warm water north of the British Isles to Scandinavia. This warm water makes western Europe have climate more like the United States instead of Canada of which it shares the same latitude range. Northern Norway is near the Arctic Circle but most of its coast remains snow/ice-free, even during the winter.

These currents show up in this section of a MODIS image of the global sea surface temperature (select the image to bring up a larger version in another window). Red and yellow indicates warmer temperatures, green is an intermediate value, and blues and then purples are progressively cooler values.

Tropical storms form over warm waters and can gather enough energy and moisture from the warm waters to turn in hurricanes (or typhoons). The ocean currents play a key role in why the southeastern United States experiences hurricanes in the summer and fall. The oceans store a lot of heat energy and release it more slowly than land or the atmosphere, so they provide a moderating influence on local climates and even global climate changes. Air circulation also affects the surface ocean currents so the air and ocean circulation are constantly playing off each other, though the ocean moves much more slowly than the air so the oceans lag behind the atmosphere. This complex interplay between ocean circulation and air circulation is a key piece of the Earth’s climate puzzle that still needs a lot of research! NASA created a nice set of videos called “Tides of Change” for Earth Science Week 2009 that provide good visualizations of the role played by the oceans in Earth’s climate. Episode 4: “Salt of the Earth” is especially good for illustrating the Gulf Stream-North Atlantic Drift.

9.3.8 Weather vs. Climate

The preceding sections have talked about weather and climate and what affects them. Weather is often confused with climate, so this section will explain the difference between them as well as what can produce changes in a planet’s global climate. The figure below illustrates the difference between weather and climate.

Weather is the ever-changing combination of winds, clouds, temperature, and pressure at a particular location and time. Climate is the long-term average of weather (usually 30 years or longer for Earth climate), which means it can change only on much longer time scales. The long-term averaging removes the chaotic wiggles making it possible to make long-term predictions of future climate. Weather predictions vs. climate predictions is analogous to not being able to predict if a particular coin toss will be heads or if it will be tails but you can easily predict the statistical results of a large number of coin tosses. Another analogy David Archer uses in his book The Long Thaw involves water in a sink with water flowing into it via a faucet and water flowing out via a drain: weather is like the ripples on the surface of the water while the average water level in the sink would be climate. Predicting the average water level in the sink is much easier than predicting all of the features of the ripples on the surface.

(For more on climate vs. weather, see the FAQ 1.2 of the Climate Change 2007: Working Group I report of the IPCC.) The hydrological (water) cycle, mountain ranges (rain shadows), and ocean currents all play key roles in determining climate on the Earth.

The Earth’s geological, paleontological, cryological, and even historical records show that climates can change but, by definition, these changes take place over decades at least on up to millennia. Careful study of the polar ice caps on Mars and other surface features show that Mars has also undergone climate changes, some drastic. Venus was once a more comfortable place but it underwent a drastic global climate change as well to become a hellishly hot and very dry place.

One of the records we have for reconstructing the temperature of the Earth to times more than 800,000 years ago are ice cores drilled into the ice sheets in Antarctica and Greenland (a cryological record). Below are temperature reconstructions from deuterium isotope abundance measurements in ice cores drilled by the EPICA group at Dome C in Antarctica (data courtesy of Jouzel, J., et al).

The records show a cyclic pattern of brief warm spells called interglacials (the peaks in the graph) separated by long cold spells, the ice ages. The pattern repeats with a period of approximately 100,000 years. Because there were no thermometers way back then, temperature reconstructions must use measurements of isotope abundances such as deuterium (hydrogen-2) or oxygen-18 to create temperature data. The isotope measurement methods use the fact that water made of oxygen-18 and/or deuterium will evaporate and condense (then precipitate) differently than ordinary water (made of hydrogen-1 and oxygen-16) because of differences in the masses of the isotopes. Water made of the lighter isotopes evaporates more easily and water made from the heavier isotopes condenses and precipitates out more readily. When the climate near the poles is colder than usual, the separation of the isotopes from the water vapor in the air is more pronounced. The result is that ice sheets are isotopically light and the oceans are isotopically heavy.

Oxygen in the oceans can get incorporated in the calcium carbonate in marine shells. Measuring the amount of oxygen-18 in fossil marine shells of different ages can give us a record of the global climate throughout time because the oceans are well-mixed over very short timescales, geologically speaking. The deuterium in ice cores and oxygen-18 in fossil marine shells give very similar results. That tells us that the data and conclusions derived from them are reliable since the very different data come from totally different places.

Another thing the plot shows is that the temperature climbs very quickly and then falls back down more gradually. The pattern is the result of subtle changes in the Earth’s orbit and tilt (described more fully below), along with complex feedback loops involving water, carbon dioxide, and other trace molecules in the climate system.

Going much further back in time, the geological records show that between 580 million and 750 million years ago, the Earth experienced some particularly severe ice ages with glaciers going all the way to the equator. A time of such extreme ice age is called Snowball Earth. Earth was able to get out of the Snowball Earth phases via climate change agents described below. Evidence for the Snowball Earth episodes comes from glacial sediments that were embedded within icebergs. The sediments of material from the middle of continents were deposited on the ocean floor far from where the glaciers would have met the oceans. Also glaciers grind up surface rock and the glaciers carry the debris along with them and deposit them in rubble piles called moraines. Rocks moving with the glaciers can scratch the surface rock layer to create long, parallel scratches. These glacier-formed features are found in places that would have been near the equator when they were created. Another piece of evidence for the Snowball Earth episodes is described in a later section about the carbon cycle.

Recalling what affects the surface temperature of a terrestrial planet, we can make a brief list of what can cause long-term climate changes: solar brightening, changes in the orbit and axis tilt, change in reflectivity, and changes in greenhouse gas abundance.

9.3.8.1 Solar Brightening

Observations of solar-type stars and our understanding of how the nuclear fusion rates in the core of the Sun change as it uses up its hydrogen in its core tell us that the Sun will gradually brighten with time. The Sun was about 30% dimmer when it formed 4.6 billion years ago than it is now. Because this process is so gradual, with effects taking tens of millions of years to be noticeable, other climate change effects described below are much more important in the short-term. These other effects have actually made Earth and Mars cooler now than they were in the past instead of hotter.

9.3.8.2 Reflectivity (Albedo) Changes

Increases in the albedo of a planet will mean less solar energy absorbed and therefore cooling of the planet. Natural ways to increase a planet’s albedo are through volcanic eruptions—the dust particles reflect the sunlight (the smaller they are, the more they reflect), increased ice coverage, and increased cloud cover. On the Earth, if the volcanic dust can reach the stratosphere, it can spread around the globe and stay aloft for several years. A natural way to decrease a planet’s albedo is through melting of ice. On the Earth, human activity has led to increases and decreases in the Earth’s reflectivity of which we are still trying to determine the net effect. Smog acts like the volcanic dust particles to increase the albedo. Deforestation also increases the albedo by removing the sunlight absorbing plants. However, building of cities and roads tend to make the surface darker, the so-called “urban heat island” effect.

9.3.8.3 Greenhouse Gas Abundance Changes

More greenhouse gases in the atmosphere usually mean more infrared energy is trapped close to the planet’s surface. A warmer surface can lead to increased evaporation or sublimation (ice directly to vapor) and therefore increase the surface air pressure. On the Earth, the increased amount of water vapor would further amplify the greenhouse effect from carbon dioxide and methane. Decreased greenhouse gases usually mean cooler temperatures and if the cooling is great enough (as found on Mars, for example), a drop in atmospheric pressure as gases freeze. Note that I inserted the word “usually” in the expected trends as a warming planet from increased greenhouse gas abundance could have increased cloud cover which may dampen or delay the warming because of the increased albedo. Clouds and dust (aerosols) create the largest uncertainties in our current climate models. Aerosols are very small droplets of material or particles suspended in the atmosphere like dust and sulfuric acid droplets but are not water droplets or ice. However, they can serve as the nuclei around which water can accumulate to make clouds, so they can have a direct effect of reflecting sunlight or an indirect effect of creating clouds that reflect sunlight (see the aerosol formation and climate page on RealClimate for more details).

Because of water’s short residency time in our atmosphere (a water molecule will be in the atmosphere for about a week) and the amount of water vapor present depends very sensitively on the temperature, water vapor is considered just an enhancer of climate change by the greenhouse effect rather than an agent of change. If another gas starts to warm a planet, the increased amount of water vapor from evaporation will approximately double the greenhouse effect of the other gas. Similarly, if a decrease in another gas cools the planet, the amount of water vapor will drop too to further weaken the greenhouse effect. In this way water vapor creates a positive feedback. A positive feedback enhances whatever initial change is made in a system. A negative feedback dampens any initial change in a system to return it back to its original state. An example in another area of experience: if you put a microphone next to a speaker, a positive feedback creates the high-pitched, very loud squeal. When you move the microphone away from the speaker to minimize the squeal, that’s a negative feedback.

Other gases like carbon dioxide have much longer residency times (decades to centuries). Their presence does not depend so sensitively on the temperature so they can be agents of change moving a climate system out of balance. In the climate change research, the long-lived gases are called radiative forcing agents while water vapor is just a feedback actor.

9.3.8.4 Axis Tilt and Orbit Changes

Gravitational interactions of a planet, its moons, and the other planets of the solar system cause the tilt of the planet’s rotation axis to change and the planet’s orbit to change shape and orientation in a periodic way. In addition, the rotation axis precesses. Put all of these agents together, all with different periods, and they can create dramatic changes in a planet’s climate by changing the insolation: the angle of sunlight hitting a given point on the planet’s surface and the amount of solar radiation reaching the planet’s surface. Adding in the other agents of climate change above can make even subtle changes in the axis or orbit much more significant.

The angle of the tilt of the Earth’s rotation axis varies about its current value of 23.44 degrees from between about 22 degrees and 25 degrees with a period of approximately 41,000 years. Larger tilt creates more extreme seasons—warmer summers and colder winters. A decreasing tilt is thought to bring about ice ages because the milder summers allow ice and snow to last throughout the year in the higher latitudes (closer to the poles). This increased ice coverage increases the reflectivity of a planet compounding the effect of the milder summers. It also appears that the amount of carbon dioxide falls as ice sheets grow, leading to further cooling. The amount of the tilt of the Earth’s rotation axis is prevented from changing too much because of the stabilizing influence of the Moon’s gravity. Mars’ rotation axis is now about 25.2, close to the Earth’s tilt, so it experiences seasons similar to ours (though roughly twice as long because of its larger orbit). Mars does not have any massive moons so it has experienced much wilder fluctuations in its rotation axis tilt throughout its history—between 13 and 40 degrees and maybe up to 60 degrees over a 100,000-year cycle (the second link brings up a book in Google books—go to p. 167). Mars is also closer to Jupiter so Jupiter’s gravity has a greater influence in perturbing Mars’ rotation axis than Jupiter has with the Earth. Recent evidence of these climate cycles on Mars can be seen in the Mars Reconnaissance Orbiter’s observations of rock layering and of layering of the polar ice caps.

Recall from the seasons section in the third chapter that the Earth’s orbit is elliptical and that it gets closest in early January. With an orbit eccentricity of 0.0167, Earth receives about 7% more solar energy in early January than at its aphelion point in early July. The Earth’s orbit changes eccentricity on cycles of about 100,000 and about 400,000 years. At greatest eccentricity, the Earth can receive over 20% more solar energy at perihelion than at aphelion. Mars has a more elliptical orbit than the Earth and as a result, its southern hemisphere winters are long and extreme while its northern hemisphere winters are short and milder.

The coordinates section of the third chapter discusses the precession of the Earth’s rotation axis that is caused by the Sun’s gravity and the Moon’s gravity pulling on the non-perfectly round Earth. The Earth bulges a bit at the equator and is flattened at the poles primarily because of its rapid spin. The Sun’s and Moon’s gravities cause the rotation axis to slowly wobble with a period of 26,000 years. Another precession is with the orbit of the Earth. The slight influences of the other planets in the solar system, primarily Jupiter and Saturn, cause the perihelion direction to shift. This reduces the cycle of when perihelion would occur from 26,000 years to about 21,600 years.

For the Earth we have an approximately 41,000-year axis tilt period plus the 100,000-year and 400,000-year cycles of variation in the Earth’s orbit shape. Add in the 21,600-year cycle of precession of the Earth’s perihelion date, you will find the net effect to be quite complex to predict. James Croll in the mid-1800s suspected that these orbit variations might cause the rise and fall of the ice ages. Later, in the early 1900s, Milutin Milankovitch quantified the changes and developed the mathematical theory behind the orbit/tilt variations and so these cycles are called Milankovitch cycles in his honor. Milankovitch’s calculations have been refined since his time but the basic idea of his theory is still valid. Past ice ages correlate well to the summer insolation at 65 deg N latitude. The northern hemisphere is more important because of the greater amount of land area in the northern hemisphere compared to the southern hemisphere—the oceans have a larger heat capacity than land and so they tend to dampen changes in the temperature.

However, the axis tilt and orbit variations cannot by themselves, explain the magnitude and quickness of the temperature changes between the ice ages and the interglacials. One has to include the effects of reflectivity changes and greenhouse gas abundance changes—the positive and negative feedback effects of them—to explain the magnitude and quickness of the temperature changes fully. For more on the Milankovitch cycles, see the Astronomical Theory of Climate Change webpage on the NOAA Paleoclimatology website (and the links therein).

9.3.9 Appearance and Colors of the Atmospheres

Most of the planet atmospheres reflect enough of the visible sunlight that only the upper layers of their atmospheres can be seen. Only the Earth and Mars have atmospheres transparent to most of the visible light so that we can see what lies below their atmospheres. I acknowledge that I have a visible light bias in this section—the other planet atmospheres are more transparent at other wavelengths outside of the visible band. If humans had eyes sensitive to certain parts of the infrared, we would probably say that Earth’s atmosphere was a thick haze that prevented photography of the surface from space.

In visible light, Venus is a bland, yellow-white planet. Venus’ atmosphere is 96 percent carbon dioxide but it is the thick cloud layer of sulfuric acid droplets that reflects back about 70 percent of the sunlight and make Venus brighter than any other object in our sky besides the Moon and the Sun (in fact Venus can be seen in broad daylight if the Earth’s atmosphere above you is very clear). Venus’ cloud layer extends from 30 kilometers to 60 kilometers above the surface. Below 30 kilometers Venus’ atmosphere is clear because the high temperature near the surface evaporates any cloud droplets that drop too far. What sunlight that makes it through the clouds, has an orange tinge to it because the blue colors are absorbed by the clouds. The sulfuric acid may be from sulfur compounds, possibly from volcanoes, that chemically react with the trace amounts of water vapor left in the atmosphere.

Two views of Venus: image on the left is in the visible band and the image on the right is in the ultraviolet band and colorized to match the visible band image. The structure of the clouds are revealed in ultraviolet light. It is the ultraviolet images that most astronomy books will use to show what Venus looks like. To see what lies below the surface, astronomers use the long wavelengths of radio. The rocky surface of Venus has been mapped using imaging radar by spacecraft orbiting Venus, such as the spectacular Magellan spacecraft that surveyed Venus in the early 1990’s.

Earth’s atmosphere is mostly transparent to visible light with a blue tint caused by the preferential scattering of blue sunlight by the nitrogen and oxygen molecules. Clouds of water droplets and ice crystals form up to about 10 kilometers above the surface. The droplets and crystals are large enough to reflect all wavelengths of visible equally, so the clouds have a white color. Because of the Coriolis effect (recall the previous section), the clouds form spiral patterns.

Earth and Mars to the same scale. Transparent atmospheres allow us to see their surfaces.Mars’ carbon dioxide atmosphere is also mostly transparent to visible light. Its thin white clouds are mostly water ice crystals. Near the poles the temperature is cold enough that carbon dioxide can freeze to form white clouds of carbon dioxide ice crystals. Some clouds have a yellow color because they are composed of fine dust particles a few micrometers across. Astronomers expected Mars’ sky to have a deep blue color as seen from the surface because the atmosphere is only one percent the thickness of the Earth’s atmosphere. However, pictures from the Mars surface landers show the martian sky to be pink from sunlight bouncing off dust particles blown off Mars’ red surface.

Two views of Mars from two generations of Mars explorers: Viking 2 lander on left and Mars Pathfinder on right.

Jupiter’s atmosphere is very dynamic and colorful and Saturn’s atmosphere is a muted version of Jupiter’s. Even though their atmospheres are primarily hydrogen and helium, the clouds of ammonia ice crystals in their upper atmospheres give the planets their appearance. The strong Coriolis effect from their rapid rotation deflect the clouds into bands parallel to their equators.

The bright zones on Jupiter are regions of upwelling convection cells in the upper atmosphere with more ammonia clouds than the darker bands. The dark bands are where we see to the warmer clouds made of ammonium hydrosulfide ice crystals about 20 to 30 kilometers below the ammonia cloud deck. Clouds of water ice crystals are thought to exist about 100 kilometers below the ammonia cloud deck, but the Galileo probe that plunged through Jupiter’s clouds in early December of 1995 found no water layer. However, this may because the Galileo probe descended through an unusually dry and cloud-free part of the atmosphere.

Saturn’s layer of clouds is about twice as thick as Jupiter’s because of the colder temperatures and lower gravity compression on Saturn. The strong jet streams in their atmospheres create turbulent eddies of swirling clouds, some several thousands of kilometers across. One spectacular example is the Great Red Spot on Jupiter—a hurricane twice the size of the Earth that has lasted for over 400 years. Such storms and the belted patterns on the Jovian planets can last so long because there is no solid surface for the storms to expend their energy.

What is puzzling about the clouds is their color. The ammonia ice clouds should be white, yet they have a variety or red, orange, yellow, and brown colors. Sunlight striking the clouds causes photochemical reactions with the molecules in the clouds. The resulting organic compounds, or trace amounts of sulfur and phosphorus may be responsible for the colors in the clouds.

Uranus and Neptune also have thick cloud decks but Uranus’ atmosphere does not have the prominent bands and storms seen on the other Jovian planets. This is because Uranus does not have an extra internal heat source like the other Jovian planets, so it does not have the convective motions in its atmosphere. Neptune’s clouds are deflected to form bands parallel to its equator because of its rapid rotation. Neptune can also have turbulent eddies form in its atmosphere. When the Voyager spacecraft flew by Neptune in 1989, it found a large dark storm, called the Great Dark Spot (very original, yes?), that was about the size of Jupiter’s Great Red Spot. However, recent Hubble Space Telescope photographs show that the Great Dark Spot seems to have dissipated.

Uranus (left), Neptune (right), and Earth to the same scale. Methane gives Uranus and Neptune their blue color. Neptune’s Great Dark Spot is seen in the center of the Neptune image.

Uranus and Neptune both have a blue color. Instead of ammonia clouds, their clouds are made of frozen methane crystals because they are much colder than Jupiter and Saturn. The red and orange colors of sunlight are absorbed by the methane in their atmospheres while the blue colors are scattered back out, producing the blue color with a faint greenish tinge.

Sections Review

Vocabulary

• albedo
• climate
• convection
• Coriolis effect
• cosmic rays
• differentiation
• escape velocity
• exosphere
• greenhouse effect
• hydrostatic equilibrium
• ideal gas law
• mesosphere
• ozone
• pressure
• solar wind
• stratosphere
• temperature
• thermosphere
• troposphere
• weather

Formulae

1. escape velocity = Sqrt[(2 × G × mass/distance)], where the mass is the mass of the planet or moon, the distance is measured from the center of the planet or moon, and G is the universal gravitational constant.
2. average gas speed = Sqrt[(3 × k × temperature/gas molecule mass)], where k is the universal Boltzmann constant.
3. general rule of atmosphere escape: if the average gas molecule speed is less than 0.2×(the escape velocity), then more than 1/2 of that type of gas will remain after 1 billion years.
4. pressure = k × number density × temperature.

Review Questions 2

1. In what ways are Jovian planets different from terrestrial planets?
2. Why are Jovian and terrestrial planets different from one another?
3. What two things determine the thickness of a planet’s atmosphere?
4. Which will have a large escape velocity: something with small surface gravity or something with large surface gravity?
5. Does a moon’s escape velocity depend on the gravity of the planet it orbits? If yes, explain how; if not, why not?
6. At a given temperature, which molecule travels fastest: a massive one or a light one? Which of the two would most likely escape from a planet’s atmosphere? Which of the two would most likely remain?
7. Which of the following things would tend to make a thick atmosphere: cold temperature, high gas particle mass, weak gravity, outgassing from the interior (volcanic eruptions)?
8. In what way does a magnetic field protect a planet’s atmosphere? In what ways is it involved with atmosphere loss?
9. Why don’t we feel a downward force on our bodies from the miles of air above us or why don’t our bodies implode from the air pressure?
10. What are two ways to increase the pressure of a gas?
11. If a gas is compressed, what is expected to happen to the temperature of the gas?
12. Explain why the pressure in your automobile tires is slightly less when they are cold than right after a long drive.
13. Why do hot air balloons float upward?
14. Why must deeper layers of an atmosphere exert more pressure?
15. How would the depth of an atmosphere change if a planet warms up or cools off?
16. What are three ways to make a planet’s average temperature greater?
17. On a planet with a thin atmosphere, what would you expect for the temperature difference between night and day to be (small, moderate, large)? Explain why.
18. What would be the expected average surface temperature of Venus, Earth, and Mars if they did not have an atmosphere? Why are their actual average surface temperatures greater than the no-atmosphere temperature?
19. If the clouds of Venus were perfect reflectors of visible light from the Sun so that no visible light reached Venus’ surface, what would happen to the surface temperature? Explain your answer.
20. If the atmosphere of Venus trapped all of the infrared light from the surface, what would happen to the surface temperature? Explain your answer.
21. What are the primary greenhouse gases in the Earth’s atmosphere? Which ones can remain in the atmosphere for decades or more?
22. What are the layers of the Earth’s atmosphere? What layers are not found on Mars and Venus and why is that?
23. In what layer of an atmosphere do clouds form?
24. Why does the temperature in our stratosphere increase with altitude and how is it beneficial to life on the surface?
25. What heats the thermosphere of a planet?
26. How are the three clouds on Jupiter and Saturn formed and what are they made of?
27. Is there expected to be any water vapor at the same elevation as the ammonia clouds on Jupiter and Saturn? Why or why not?
28. Is there expected to be any ammonia vapor at the same elevation as the water clouds on Jupiter and Saturn? Why or why not?
29. Why are the cloud decks of Saturn found at greater depths than on Jupiter?
30. Why is there so much convective motion in many planet atmospheres and some planet interiors?
31. Why do low-pressure storms develop cyclonic spirals, and why are the patterns in the two hemispheres mirror images of each other?
32. Suppose the atmosphere circulation on the Earth were stopped. What would be the effect on the temperature of the atmosphere near the equator, at the mid-latitudes, and near the poles?
33. If the Earth rotated faster, would the Coriolis effect be greater or less than what it is now? Explain your answer by comparing what would happen to a rock thrown from the north pole and landing on the equator on a rapidly spinning Earth with that thrown on a slowly spinning Earth.
34. How do clouds affect the surface temperature of a planet?
35. On Earth a molecule of water leaves the ocean. Describe the possible paths the molecule could take to return to the ocean.
36. How do mountains affect a location’s climate, particularly its annual amount of precipitation?
37. How do ocean current affect a location’s climate?
38. What is the difference between weather and climate?
39. If we cannot accurately predict the weather two weeks from now, how can we possibly predict the climate 50 years from now?
40. How has the temperature of the Earth changed over the past million or so years and how do we know?
41. What five things can cause long-term climate changes?
42. How has the Sun’s brightness changed since it formed?
43. If water vapor is the dominant greenhouse gas on Earth, why is there such a concern about other greenhouse gases such as carbon dioxide or methane?
44. What is a “positive feedback”? What is a “negative feedback”?
45. What is responsible for triggering the ice ages on Earth?
46. Is it possible for a planet’s rotation axis and orbit shape to change? What affect would those changes have on a planet’s average global temperature?
47. In what ways are the atmospheres of the terrestrial planets like each other? In what ways are they different from each other?
48. How are the atmospheres of Jupiter and Saturn different than the atmospheres of Uranus and Neptune?
49. What are visible clouds of the Jovian planets made of and why are they different from each other?

9.4 Magnetic Fields

Some planets have a magnetic field that acts like there is a giant bar magnet in the center of a planet (there isn’t really a giant bar magnet, though). The magnetic field can be aligned differently than the rotational axis. For example, the Earth’s magnetic field is tilted by about 18° with respect to our rotation axis, so compasses point to a magnetic pole that is in northern Canada.

A planet’s magnetic field forms a shield protecting the planet’s surface from energetic, charged particles coming from the Sun and other places. The Sun is constantly sending out charged particles, called the solar wind, into the solar system. When solar wind particles run into a magnetic field, they are deflected and spiral around the magnetic field lines. Magnetic “field lines” are imaginary lines used to describe the direction charged or magnetic particles will move when responding to a magnetic field. In the same way, gravity “field lines” point to the center of an object producing the gravity. You can see the direction of an ordinary household magnet’s field lines by sprinkling tiny iron filings around a magnetic—they will tend to bunch up along particular magnetic field lines.

Most of the solar wind gets deflected around the planet but a few particles manage to leak into the magnetic field and become trapped in the planet’s magnetic field to create radiation belts or “charged particle belts.”

9.4.1 Aurorae

This nice picture is from Jan Curtis’ Aurora Page. Select the link to see more of his aurora photography. I have images of aurorae too in the Beautiful Earth photo album.

One glorious effect seen when the solar wind interacts with a planet’s magnetic field is aurorae. Aurorae are shimmering light displays produced by molecules in the upper atmosphere. Fluctuations in the solar wind can give energy to the trapped charged particles in the belts. Particles with enough energy can leave the belts and spiral down to the atmosphere to collide with molecules and atoms in the thermosphere of a planet. These collisions excite the atmosphere molecules (bumping their electrons to higher energy levels). The electrons then release the excess energy as they hop downward back toward the atomic nuclei. The glow of the aurorae is the emission line spectra produced by the electrons in the rarefied gas dropping back down to lower atomic energy levels.

Aurorae seen from the Space Shuttle courtesy of NASA. Notice the colors of the aurorae at different altitudes and the large gap between the aurorae and the surface. More aurorae from space are available at the Gateway to Astronaut Photography of Earth (choose “Aurora” for the Geographic Region)Aurorae in the Earth’s atmosphere occur many tens of kilometers above the surface and pose no threat to life on the surface below. They make some spectacular displays that look like shimmering curtains or spikes of different colors of light. The magenta colors are produced by nitrogen molecules at the lower end of the aurorae (up to 100 kilometers above the surface), between 100 and 200 kilometers above the surface excited oxygen atoms produce the green colors and ionized nitrogen atoms produce the blue colors, and greater than 200 kilometers above the surface oxygen atoms produce the deep red colors. In the northern hemisphere, the aurorae are called aurora borealis or “the northern lights” and in the southern hemisphere, they are called aurora australis or “the southern lights.” For further information (and photos!), explore these sites (all will appear in another window):

• The astronomy department at Rice University (Houston, TX) has a more in-depth website on the interaction of the Earth’s magnetic field with the solar wind called Space Weather.
• The IMAGE web site discusses NASA’s mission dedicated to imaging the Earth’s magnetosphere.
• The Aurora: Forecasts and Information website from the Geophysical Institute at the University of Alaska in Fairbanks is another nice site with video, images, and information about aurorae for younger and older audiences.
• Geospace Environment Data Display System gives near-real time display of what’s happening in the sky over Poker Flat Research Range.
• Space Weather Now site from the Space Environment Center gives current images of the Sun, auroral oval images at each pole from the National Oceanic and Atmospheric Administration Polar-orbiting Operational Environmental Satellite (NOAA POES), current solar wind measurements, and a lot more (updated every few minutes).

9.4.2 Magnetic Dynamo Theory

Planets do not have giant bar magnets in their cores, so what produces the magnetic field? Recall from the beginning of the Electromagnetic Radiation chapter, that a magnetic field can be produced by circulating electrical charges. A theory called the magnetic dynamo theory says that the magnetic field is produced by swirling motions of liquid conducting material in the planet interiors. Materials that can conduct electricity have some electrical charge that is free to move about. Such materials are called metallic and are not necessarily shiny solids like copper, aluminum, or iron. Jupiter and Saturn have a large amount of hydrogen that is compressed so much it forms a liquid. Some of that liquid hydrogen is in a state where some of the electrons are squeezed out of the atoms and are free to move around.

A moving charge will produce a magnetic field. The liquid conducting material in a planet’s interior can be made to swirl about if the planet is rotating quickly enough. The faster a planet rotates, the more the material gets stirred up and, therefore, the stronger the generated magnetic field. If the liquid interior becomes solid or if the rotation slows down, the magnetic field will weaken. So in summary what a planet needs in order to produce a strong magnetic field is (1) a liquid conducting (metallic) interior and (2) rapid rotation to get the conducting material moving about. For the terrestrial planets plate tectonics may also play a role. Plate tectonics cools the planet’s mantle creating a large enough temperature difference between the core and mantle to produce convection in the metallic core needed to make a magnetic field. Let’s see how this theory explains the presence or lack of a magnetic field on some of the planets:

1. Venus has no magnetic field (or one so weak, it hasn’t been detected yet). It probably has a liquid conducting interior for a couple of reasons:

(a) Since it is almost the size of the Earth, its interior should still be very warm. Larger planets lose their heat from formation and radioactive decay more slowly than small planets. A planet with a larger volume than another planet of the same composition will start off with a larger supply of heat energy. In addition, the heat in a large planet’s interior has a great distance to travel to reach the planet’s surface and the cold outer space.

The rate of heat loss increases with the surface area. A planet with a larger surface area than another planet with the same internal temperature will have a larger rate of heat loss. The time it takes for a planet to cool off depends on the total amount of heat stored/rate of heat loss or (its volume)/(its surface area). Recall from the planet volume section that the volume increases as the diameter³. The surface area increases as only the diameter², so the planet’s cooling time increases as diameter³/diameter²= diameter. Even though its heat loss rate is greater, a larger planet has a much larger amount of energy stored in it and, thus, it will take longer to cool off than a smaller planet. Venus should have a iron-nickel core that is still liquid like the Earth’s.

(b) High resolution radar imaging of Venus’ surface by the Magellan spacecraft shows several places where volcanoes have erupted recently and produced large lava flows. Recent infrared imaging of Venus’ surface by the Venus Express spacecraft shows material that has just come out of some shield volcanoes, so Venus is still active.

The reasons why Venus does not have a global magnetic field are that it spins very s-l-o-w-l-y (about once every 243 Earth days!) and the absence of convection in the liquid core (probably because of the lack of plate tectonics for the past half billion years).

2. Mars has an extremely weak magnetic field but for a different reason than Venus. Mars is about half the diameter of the Earth and has about 1/10th the Earth’s mass, so its internal heat should have disappeared to space long ago. So even though Mars spins quickly (once every 24.6 hours), its metallic core is solid—the charges are not able to swirl about. Mars’ crust is also probably too thick for plate tectonics to occur even if the core had not cooled.

3. Earth has a strong magnetic field because it spins fast (once every 23.93 hours), it has a liquid conducting core made of liquid iron-nickel, and it has plate tectonics.

4. Jupiter has a HUGE magnetic field. Jupiter has a large amount of hydrogen that is super-compressed to form the strange liquid called liquid metallic hydrogen. This material cannot be produced on the Earth because the super-high pressures needed to squeeze some of the electrons out of liquid hydrogen cannot be produced. Jupiter also spins very quickly—one rotation in under 10 hours! Jupiter’s magnetic field is so large that from the Earth it has an angular size over four times the size of our Moon. One of the first radio sources detected from space was Jupiter. Charged particles in Jupiter’s radiation belts that are energized by the solar wind spiral around magnetic field lines to produce electromagnetic radiation in many different frequency bands. For Jupiter a lot of this energy is in the form of radio. In addition, there is a flux tube of electrical current of millions of amps flowing between Jupiter and its very geologically-active moon, Io. As described in the Jupiter moons section, Io has a number of volcanoes erupting all of the time. Particles in Jupiter’s radiation belts collide with the volcanic dust particles and charge them, further adding to the radiation belts. Charged particles flowing back and forth produce aurorae larger than the entire Earth. Spacecraft sent to Jupiter have to be specially “radiation-harded” to protect their electronics from the charged particles in Jupiter’s radiation belts.

5. Mercury is a bit surprising because it has a weak magnetic field. Mercury is the smallest of the terrestrial planets, so its interior should have cooled off long ago. Also, Mercury spins slowly—once every 58.8 days. Mercury’s high density tells us that it has a proportionally large iron-nickel core. Its magnetic field implies that Mercury’s interior is probably partially molten. In mid-2007 astronomers announced independent evidence in favor of a molten core for Mercury. Using very careful observations of Mercury’s rotation, they found that Mercury’s core could not be solid (see the next section for more on this technique).

Mercury’s situation was a major challenge to the magnetic dynamo theory. In true scientific fashion, the theory made a testable prediction: Mercury should have no magnetic field or one even less than Mars’ one because its core should be solid. Observation, the final judge of scientific truth, contradicted the prediction. Should we have thrown out the magnetic dynamo theory then? Astronomers were reluctant to totally disregard the theory because of its success in explaining the situation on the other planets and the lack of any other plausible theory.

So most take a more conservative route: either modifying the magnetic dynamo theory or investigating Mercury more closely to find out what is so unusual about its interior to produce a magnetic field despite our expectations. Is their reluctance a violation of the objectivity required in science? Perhaps, but past experience has taught that when confronted with such a contradiction, nature is telling you that you forgot to take something into account or you overlooked a crucial process. The MESSENGER mission confirmed the existence of Mercury’s liquid core through careful tracking of the spacecraft’s motion in Mercury’s gravity field. The Mercury spin data and gravity field measurements have solved part of the problem (part of the core is molten), but how has the core remained molten and convecting (even partially) despite Mercury’s small size? MESSENGER also found that the magnetic field is offset toward the north pole by about 20% of Mercury’s radius. This offset suggests that Mercury’s dynamo is created near the core-mantle boundary.

Another example of this conservative route is the discovery of the planet Neptune. When its near twin planet, Uranus, was discovered, astronomers were very confident in Newton’s gravity theory because of its over a hundred-year success rate in explaining the motions of many different types of objects. So they applied Newton’s gravity theory to Uranus’ orbit. However, after several decades of further observation of Uranus, the predicted orbit was significantly different from the observed orbit. Rather than throwing out Newton’s gravity theory, astronomers used the contradiction to predict the presence of another planet beyond Uranus. Within a couple of years, Neptune was discovered at the position predicted! Might the same sort of thing be happening with the magnetic dynamo theory and Mercury’s magnetic field? Perhaps. One thing is for sure, scientists love a good puzzle and will work hard at trying to solve it.

Vocabulary

• aurorae
• magnetic dynamo
• solar wind

Review Questions 3

1. What happens to the charged particles in the solar wind when they reach a planet’s magnetic field?
2. What happens to some of the solar wind particles that are deflected toward a planet’s magnetic poles?
3. What does the fact that aurorae are from emission lines tell you about the density of the gas where they are produced?
4. What two basic things are needed to create a magnetic field? For a terrestrial planet, what might be a third thing needed?
5. Why does the Earth have a magnetic field but its companion, the Moon, has no detectable magnetic field?

9.5 Planet Interiors

Terrestrial planet interiors are composed of three basic parts: a dense, metallic core, a lower density solid mantle surrounding the core, and a solid outer shell called the crust. Jovian planets have a core and a fluid mantle that merges smoothly into their atmospheres.

I have already discussed several observations and techniques you can use to get initial clues of what a planet’s interior is like. I will summarize them and then discuss ways to increase the accuracy of those rough initial models. Methods and observations already mentioned are:

1. The planets have flattened spherical shapes from the action of gravity and the centrifugal effect of their rotation. In the Planet Properties table (appears in a new window) the amount of the rotational flattening is the oblateness. The amount of flattening also depends on the fluidity or elasticity of the interior’s material.
2. Compute the overall average density of the planet.
3. Compare the surface material density with the overall density to find out how much differentiation has occurred.
4. Determine the composition of surface and atmosphere material from landers or remotely with spectroscopy.
5. The presence of a magnetic field requires the interior to have a liquid metallic component.

Astronomers have several other tools to probe the interiors of planets. By carefully observing the rotation of a planet, you can detect the precession (wobbling) of its rotation axis (the precession of the Earth’s rotation axis is discussed in the coordinates section of the “Astronomy Without a Telescope” chapter). The rate of precession depends on a parameter called the moment of inertia which tells you how much the mass is concentrated toward the center. The Earth’s core is considerably denser than its surface. The Jovian planets have even greater concentration of their mass at their cores than the Earth does. Tiny twists in a terrestrial planet’s spin can be used to determine if the core is solid or liquid. For example, the tiny twists in Mercury’s spin rate are too great to be caused by a completely solid core, so its core (or parts of it) must be liquid. You can illustrate this by comparing the spin of a raw egg and the spin of a hard-boiled egg—you will notice that the raw egg’s spin slows down because of the fluid inside sloshing about.

The mass distribution of a planet can be probed by observing the motions of satellites (moons or spacecraft) in the planet’s gravitational field. Mass lumps in the surface layer can be detected, as well as, asymmetries in the overall mass distribution. For example, the center of the Moon’s mass is 2 kilometers closer to the Earth than the center of its overall shape (the geometric center). The Moon’s crust on the Earth-facing side is several kilometers thinner than the crust on the far side. This is probably a remnant of the Earth’s gravity acting on the early Moon’s molten interior billions of years ago. Mars’ center of mass is north of the geometric center. This is associated with the fact that Mars’ southern cratered highlands stand about four kilometers higher than the northern volcanic plains.

Planet interiors are warmer than outer space, so energy will flow outward from the interior toward the surface. There are three possible sources of heat in a planet’s interior. One is the heat of accretion during its formation—dust, pebbles, and rocks stuck together as they were gravitationally attracted together in the planet’s formation, releasing gravitational potential energy. A second source is during differentiation—loss of gravitational potential energy as the denser material sunk to the core. Both of those sources occur early in a planet’s history. The primary source today, billions of years later, is the heat from radioactive decay of unstable atomic nuclei: uranium-238, uranium-235, thorium-232, and potassium-40. The heat in a terrestrial planet’s rock/metal interior is not from the great pressures as happens in a gas because the rock/metal are compressed only slightly—a chunk of granite from the surface would be compressed to half its size at the base of the mantle. Rock in the deep interior will eventually become cold at the same high pressure as before.

Energy flows from hot to cold places through radiation (glowing), convection (bulk motions of the fluid), and conduction. Radiative energy transfer is not used in rock because light cannot pass through it, convection occurs in the core and mantle, and conduction occurs very slowly in the crust and the top of the mantle below the crust and is much faster in the metallic core. Radiative energy transfer happens at the surface where the infrared energy photons can travel with much greater ease through the atmosphere to space.

The rate of heat loss from the warm interior and the rate at which the temperature increases at greater depths closer to the core are important parameters for determining the interior structure. On the Earth, scientists can drill several kilometers into the crust and measure the temperature difference between the bottom of the hole and near the top. For the Jovian planets, infrared telescopes are able to detect their large heat flows.

For the terrestrial planets, the most useful data comes from seismology—the study of the interior from observations of how seismic waves (“planetquake” waves) travel through the interior. Seismic waves slightly compress rock or cause it to vibrate up and down. They are produced when parts of a planet’s crust suddenly shift and can be felt on the surface as a quake. These quakes have been studied extensively on the Earth, so I will focus on the use of earthquakes in what follows but remember that the same principles can be applied on any solid surface where instruments, called seismometers, are placed to study the jolts. Seismometers were left on the Moon by the Apollo astronauts and the Viking 2 lander on Mars had a working seismometer but persistent buffeting by the martian winds prevented it from being able to definitely detect any marsquakes (the seismometer on Viking 1 did not work). No seismometers have been placed on Venus or Mercury.

The speed, amplitude, and direction the seismic waves move depend on the particular type of wave and the material they pass through. Just as a physician can use an ultrasound to get a picture of your anatomy or of a fetus, you can use seismic waves to get a picture of the Earth’s interior (though it is a bit cruder than the physician’s ultrasound). Earthquakes will produce two main types of waves: P (pressure) waves and S (shear) waves.

P waves are like sound waves—matter in one place pushes against adjacent matter compressing it. The result is a series of alternating stretched and compressed rock propagating in the same direction as the compression. It is like what happens when you stretch out a Slinky horizontally on a long table and give one end a sudden horizontal shove. You will see a wave of compressed metal coil move across the length of the Slinky to the other end. P waves can travel through solid and liquid material and move faster than S waves.

S waves are like waves in a jerked rope—matter moves up and down or side to side. Liquid matter prevents S waves from spreading. Timing of the arrival of seismic waves from at least three stations in a triangular array allows the earthquake center to be located. Seismometers on the opposite side of the Earth from the earthquake detect only P waves so there must be liquid material in the Earth’s core. The size of the liquid core can be constrained from how far away a seismometer can be and still detect both S and P waves.

Seismic waves refract (bend) inside the Earth because of the change in speed of the waves as they move through material of variable density, composition, and temperature. Abrupt changes in direction occur at the boundary between two different layers. P waves entering the core are bent toward the Earth’s center so they only reach the part of the Earth’s surface opposite the earthquake. There is a shadow zone between the P waves that pass through the mantle only and those that pass through the mantle and the core. The shadow zone location also puts constraints on the size of the liquid core. It has a radius of about 3500 kilometers and is made of an iron-nickel alloy with a small percentage of sulfur, cobalt, and other minerals and has a density of around 12 (water = 1). Very weak P waves are felt in the shadow zone, indicating that a smaller solid component resides at the very center with a radius of about 1300 kilometers and a density around 14. Even though the temperature of the interior increases toward the center (it is about 6300 K at the center), the high pressures in the inner solid core (up to 3.64 million bars at the center) make it solid while the outer metallic core remains liquid.

The Earth’s mantle is made of hot but not quite molten iron-rich silicate and magnesium minerals like olivine, pyroxene, modified spinel, spinel, perovskite (primarily crystals of a dense form of magnesium silicate), and postperovskite (even denser magnesium silicate) and is around 2900 kilometers thick. Geologists further sub-divide the mantle into an upper mantle below the crust about 625 kilometers thick and a lower mantle 2300 kilometers thick. The upper mantle is made of in order of increasing depth olivine and pyroxene, modified spinel, and spinel layers. The lower mantle is made of a perovskite layer 2000 kilometers thick on top of a thinner 300-kilometer thick postperovskite layer just above the core. The density increases from about 3.5 below the crust to over 5 at the core boundary. The temperature at the top of the mantle below the crust is about 1700 K to about 3100 K at the base of the mantle—not hot enough to liquify the rock at the pressures inside the mantle (about 1.36 million bars at the base of the mantle next to the core). Even though the mantle is not liquid, it can deform and slowly flow when stressed. Convective motions in the mantle rub on the crust to produce earthquakes and volcanoes. This convective motion is very slow compared to a human lifetime, it can take several tens of millions of years for a chunk of rock to move from the inner boundary of the mantle to just below the crust.

9.5.1 Putting the pieces together

From these observations and knowledge of the physical laws of nature, astronomers can construct and refine computer models of the planet interiors. The three parameters of pressure, density, and temperature determine the state of the material and a relation called the equation of state relates the three parameters to each other. It tells what the density is for a given pressure and temperature. For rock, the equation of state is complex and somewhat uncertain. Rocky materials of different compositions and phase or state of the material (solid, liquid, gas) all will have different equations of states. (In contrast, the equation of state for stellar interiors is fortunately much simpler—see the Stellar Structure chapter for a discussion of the equation of state in stars.)

Planets exist in a balance between the compression of gravity and the pressure of the liquid and solid. Deeper layers experience more compression from the overlying material so the balancing outward pressure must increase. (This principle can also be applied to the gas of atmospheres to show why the atmosphere is thicker closer to the surface.) The computer model calculates the density in each layer from the equation of state with appropriate values of the temperature at that depth. The computer program starts off with the observed surface conditions and layer-by-layer, works its way toward the center. If the model does not arrive at a value of the total planet mass by the time it reaches the center, it must be revised. The models are checked against the other observables described above (moment of inertia, oblateness, gravity field measurements, heat flow, etc.) and refined further.

What follows is a brief description the other planet interiors found from putting all of the observations and theory together (see also the figure below). Mercury has a very large iron core about 3500 kilometers in diameter that makes up 60% of its total mass) surrounded by a silicate layer only 700 kilometers thick. Its core is partially molten. Its core must also contain some lighter element such as sulfur to lower the melting temperature. Venus’s interior is very much like the Earth’s except its iron-nickel core probably makes up a smaller percentage of its interior. Mars has a solid iron and/or iron-sulfide core 2600 to 4000 kilometers in diameter, surrounded by a silicate mantle and rocky crust that is probably several hundred kilometers thick.

The crust and the outermost part of the mantle make up a layer of hard rock called the lithosphere. The rigid, brittle lithosphere gradually turns into the softer, more pliable (and hotter) asthenosphere. Small planets have very thick lithospheres that extend from the surface to almost the core or all the way to the core. Large planets will have thin lithospheres because they still retain a lot of heat. Earth’s lithosphere is thin enough to be cracked into chunks called “plates” that will discussed in detail on the following webpage. The Earth’s lithosphere is on average about 100 kilometers thick, with the oceans plates being thinner than the continental plates. Venus’ lithosphere is a little thicker than the Earth’s. Mars’ lithosphere extends hundreds of kilometers almost all the way to its core. Mercury’s lithosphere is 700 kilometers thick.

The Jovian planets are made of lighter materials that exist under much higher pressures than can occur anywhere on the Earth. Direct observations of their structure are still limited to the top several hundred kilometers of their atmospheres. Using those observations, computer models are calculated to predict what their interiors are like. Jupiter’s hydrogen, helium atmosphere is at least 1000 kilometers thick and merges smoothly with the layer of liquid molecular hydrogen. The liquid hydrogen layer is about 20,000 to 21,000 kilometers thick. The pressure near the center is great enough to squeeze electrons from the hydrogen atoms to make the liquid metallic hydrogen layer that is around 37,000 to 38,000 kilometers thick. Jupiter probably has a silicate/ice core twice the diameter of the Earth with about 14 times the Earth’s mass. Although the core is made of silicates and ices, those materials are much different than the silicates and ices you are familiar with here on the Earth because of the pressures that are many times greater than the pressures at the Earth’s core and temperatures in the 20,000 to 30,000 K range. Saturn is a smaller scale version of Jupiter: silicate core 26,000 kilometers in diameter, ice layer (solid methane, ammonia, water, etc.) about 3500 kilometers thick beneath a 12,000-kilometer thick layer of liquid metallic hydrogen, liquid molecular hydrogen layer around 28,000 kilometers thick, and atmosphere about 2000 kilometers thick.

The compression on Uranus and Neptune is probably not enough to liquify the hydrogen. Uranus and Neptune have silicate cores 8000 to 8500 kilometers in diameter surrounded by a slushy mantle of water mixed with ammonia and methane around 7000 to 8000 kilometers thick. This mantle layer is probably responsible for their strange magnetic fields which are not centered on the planet centers and are tipped by large degrees from their rotation axes. At the top is the 9000 to 10,000-kilometer thick atmosphere of hydrogen and helium. Tiny Pluto probably has a rocky core half its size surrounded by an ice mantle/crust.

Vocabulary

• equation of state
• lithosphere
• seismology

Review Questions 4

1. What methods of determining the structure of planet interiors can be done from Earth-bound observations? What methods require observations by spacecraft flying by a planet or in orbit around it? What methods require spacecraft to land on the surface or travel into the planet’s interior?
2. Suppose spacecraft are orbiting two identical-looking planets having no natural moons. The spacecraft orbiting the first planet has a perfectly elliptical orbit and the spacecraft orbiting the second planet has a nearly elliptical orbit but with small accelerations. What does this tell you about the interior of the two planets?
3. Where does the heat inside a planet come from and how does it eventually leak out to space?
4. Why is the placement of seismometers on a planet’s surface so beneficial for understanding the planet’s interior?
5. Why will seismology never be possible on the Jovian planets?
6. Why do small terrestrial planets have thicker lithospheres than larger terrestrial planets?
7. Compare and contrast the structure of Jupiter with that of the Earth.
8. Compare the thicknesses of Saturn’s various layers to the corresponding layers in Jupiter.
9. How are the interiors of Uranus and Neptune different from the interiors of Jupiter and Saturn?

9.6 Planet Surfaces

Terrestrial planets have hard surfaces that can be re-shaped by several different processes: impact cratering, volcanism, erosion, and tectonics.

9.6.1 Impact Cratering

There are still small chunks of rock orbiting the Sun left over from the formation of the solar system. Some of them have orbits that cross the orbits of the planets and moons. When they get close enough to a planet or moon, they will be pulled in by the large body’s gravity and strike the surface at a speed of at least the escape velocity of the planet or moon, i.e., faster than a bullet. At such speeds, the projectile explodes on impact and carves out a round bowl-shaped depression on the surface. This process is impact cratering. How can you distinguish an impact crater from a volcanic crater?

Volcano craters are above the surrounding area on mountaintops while the craters from impacts are below the surrounding area with raised rims. The craters on all of the moons except Io, Mercury, and most of the ones on Mars are from impacts. The kinetic energy of the impacting meteorite or asteroid is converted into heat, sound, and mechanical energy—the projectile explodes on impact. The explosion is what carves out the crater so almost all craters are round (otherwise the great majority would be oblong in shape). See the “Not Round” page from the THEMIS site for what can make an impact crater not round (links will appear in a new window).

The rock on the surface of the planet or moon is bent backward, upward, and outward so the amount of material ejected is much larger than the projectile. Large craters will have a central peak formed by the rock beneath the impact point rebounding upward and they may also have terracing of the inner walls of the crater from the collapsing of the crater rim inward. The size of the craters having central peaks depends on the gravity of the planet or moon: on the Moon craters larger than about 60 kilometers in diameter have central peaks while the crater diameter on the Earth needs to be larger than just 1 to 3 kilometers.

Impact cratering was especially prevalent for the first several hundred million years after the planets formed as the planets swept up left-over material. The last stage of that “sweeping up” is called the late heavy bombardment and it occurred from about 4.1 to 3.8 billion years ago. Impacts as large as the one that led to the demise of the dinosaurs in much more recent history were happening about once a month. Most of the impact basins—craters measured in hundreds of kilometers—were made during this time. It is noteworthy that about the time the heavy bombardment ended, life took hold. The oldest fossil evidence of ancient organisms dates back to 3.5 billion years ago and evidence for biological activity based on isotopic ratios of carbon may date back to about 3.85, even up to 4.2 billion years ago, though the carbon isotope ratio evidence is controversial.

The number of craters per unit area on a surface can be used to determine an approximate age for the planet or moon surface if there is no erosion. The longer the surface has been exposed to space, the more craters it will have. If you know how frequently craters of a given size are created on a planet or moon, you can just count up the number of craters per unit area. This assumes, of course, that the cratering rate has been fairly constant for the last few billion years. The heavy bombardment of about 3.8 billion years ago must be taken into account when using the crater age dating technique. For example, the highland regions on the Moon have ten times the number of craters as the maria, but radioactive dating (explained in the next chapter) shows that the highlands are approximately 500 million years older than the maria, not ten times older. At a minimum crater-age dating can tell you the relative ages of surfaces (which surface is older than another). Careful studies of how the craters overlap other craters and other features can be used to develop a history or sequence of the bombardment on the moons and planets.

All bodies with hard surfaces have impact craters. Worlds with less volcanism or erosion or tectonic activity in their history will retain more impact craters since the planet formed. Worlds with more geological or erosional activity will have newer surfaces or craters that have been so worn away as to be unrecognizable. Earth has over 170 impact craters on its continents with the 1.2-km diameter Meteor Crater in northern Arizona being one great, well-preserved example. Even Venus with its thick atmosphere has impact craters, though they all have diameters measured in kilometers because smaller projectiles burn up in its atmosphere.

The first frame of a 3D movie flyover of Tycho Crater on the Moon as seen by Kaguya. Tycho is 85 kilometers wide. It displays the classic terraced crater walls and central peak of a complex crater. Select the image to go to the Selene website for a great flyover of the crater. LPI website with movie link

Victoria Crater on Mars as seen by the Mars Reconnaissance Orbiter. Victoria is 800 meters in diameter. It is a simple crater that has been partially filled in with sand—note the sand dunes on its floor. If you download the full-resolution version of this picture from the link on the picture, you will be able to see the rover, Opportunity next to the top left crater rim.

Cunitz Crater is the 48.5-km diameter crater in the foreground of this 3D perspective view of Eistla Regio on Venus as seen by Magellan. In the distance is the 3-km high volcano Gula Mons.

9.6.2 Volcanism

Volcanism is any eruption of molten lava onto the surface. The molten rock has a lower density than solid rock so it rises. Also the pressure from the surrounding solid rock squeezes the molten rock upward. Molten rock contains trapped gases that expand as it rises causing it to rise even faster. The structures that result from erupting lava depend on the thickness and density of the lava.

Very thick, low-density lava can make steep-sided stratovolcanoes like the Cascade volcanoes in Oregon and Washington and Mt Fuji in Japan. The lava does not flow as far as the other two types, so their diameters are not as great as the other types. They are common on the Earth. The “pancake domes” on Venus may be considered a type of stratovolcano but their shapes are much more cylindrical without a peak.

Mt Shasta is near the southern end of the Cascade Range of volcanoes

If the lava is made of thinner, higher density material called basalt, the volcanoes will be much wider in diameter. Thicker basalt will make shield volcanoes like the Hawaiian volcanoes, Sif Mons on Venus, and Olympus Mons on Mars. These types of volcanoes are found throughout the solar system. Besides being very wide in diameter, shield volcanoes can also be very tall—from its base below the ocean surface to its peak, Mauna Kea and Mauna Loa are taller than Mt Everest. Olympus Mons is about three times taller than Mt Everest (24 km above the surrounding plains) and would cover up most of Texas (about 600 km in diameter).

The thinnest, runniest basalt will make lava plains that can re-shape the surface by covering up things. Lava plains are found here and elsewhere in the solar system. For example, large lava flows covered up most of modern-day eastern Washington and northeastern Oregon with a layer more than 1.5 kilometers thick. The dark maria on the Moon are basalt lava plains that filled in giant impact basins approximately 3.5 billion years ago. Mercury has lava plains formed after the late heavy bombardment when it was still geologically active. Much of Venus is covered in lava plains.

Volcanism also adds gases to a planet’s atmosphere either explosively during a volcanic eruption such as Mt. St. Helens in Washington or Mt Pinatubo in the Philippines, or more quietly as in the volcanic vents in Hawaii and Yellowstone in Wyoming (Yellowstone pictures link). This outgassing is what created the terrestrial planets current atmospheres (Earth’s has undergone further significant processing through the action of life).

9.6.3 Erosion

Erosion is the breaking down or building up of geological structures and transporting of material (rock for our purposes here) by ice, liquid, or wind. Water ice in the form of glaciers carves out bowl-shaped valleys such as Yosemite in California and those in Glacier National Park in Montana. Glacial moraines form from gravel transported by glaciers. The gravel comes from the glacier scraping rock and it gathers along the side and front of the glacier. When the glacier recedes the gravel piles from the side and front of the glacier remain. One classic example is Wallowa Lake in the northeastern corner of Oregon. Liquid water on the Earth has carved through layer upon layer of rocks to produce such spectacular things as the Grand Canyon on Earth. Valleys carved by liquid water have much sharper V-shapes than glacial valleys. Erosion by liquid water on Mars in the past is responsible for river drainage channels and gullies on the walls of impact craters as illustrated by this particularly sharp image taken of the side of Newton Crater by the Mars Global Surveyor. The crater rim is on the right and the crater floor is on the left.

On Titan, the largest moon of Saturn, liquid methane has carved features into the ground made of frozen water that are quite reminiscent of features on Earth. Below is a synthetic aperture radar image from Cassini of a canyon system on Titan. Fluids flowed from high plateaus on the right to lowland areas on the left. All of the tributaries suggest that methane rainfall is effectively eroding the surface.

Wind picks up small sand particles on a planet’s surface and strikes it against a hard surface to chip pieces of it away (to make more sand!). The winds can also shape hills of sand into sand dunes as seen on the Earth, Mars, and Titan. Image below links to originals (and much larger versions): Mars dunes—Titan dunes.

The agents of erosion can, of course, work together. Water expands when it freezes so when it trickles into cracks in rocks and freezes during the winter, it can enlarge the cracks enough to separate the minerals from each other. (Plant roots do the same thing as well.) Liquid water from rainfall and snowmelt takes those minerals down to the oceans—a process which also helps to remove carbon dioxide from the atmosphere. Wind whittles away the exposed rock left behind and the result can be beautiful formations like Bryce Canyon in Utah.

Besides chipping away at mountains and other rises in the land bit by bit, erosion can also add to formations by depositing sediments when liquid slows down enough for the suspended sediments to sink to the stream or river bed such as the Mississippi River Delta and the Lena River Delta in Russia (shown below). Fossilized river deltas are seen on Mars.

9.6.4 Tectonics

Tectonics is any stretching or compression of the lithosphere. Usually tectonic activity is due to convection in the mantle below the lithosphere but it can occur when an especially massive landform such as a volcano makes the lithosphere under it buckle from its weight or a rising plume of extra hot material in the mantle pushes on the lithosphere overhead to make a bulge such as coronae features on Venus and possibly the Tharsis Bulge on Mars (though that one may be due to mantle convection long ago).

Aine Corona on Venus as seen by Magellan. Aine Corona is approximately 200 kilometers across. Just north of Aine is a 35-km diameter “pancake” dome feature made from extremely viscous (thick) lava. Another pancake dome is inside the western ring of the corona’s fractures. Pancake domes are a type of volcanic structure seen only on Venus.

Tharsis Bulge side of Mars as seen by the Viking 1 orbiter shows the true “Grand Canyon” of the solar system, Valles Marineris, a huge fracture that probably formed as that side of Mars was raised up. Valles Marineris would stretch across the entire United States with a bit left over. To the west (left) are large shield volcanoes on Tharsis each about 25 kilometers high. Select the “Valles Marineris” link to the Mars Odyssey THEMIS website for an excellent flyover of Valles Marineris.

Mercury has high cliffs (as tall as 3 km) called “lobate scarps” that run for hundreds of kilometers that are probably the result of the shrinking of Mercury as it cooled from its formation. Recall that Mercury has the proportionally largest metallic core of the planets. As the metallic core cooled, it shrank and the mantle and brittle crust would have had to shrink too, crinkling in the process. The image below from MESSENGER’s first flyby of Mercury indicates that the volcanic flows in Mercury’s early history occurred while the crinkling was going on. The two smaller white top arrows point to a scarp that formed after the lava plain flow had filled in a number of the craters. The bottom three larger white arrows point to a possible older scarp against which the lava plain flow stopped. The black arrow points to a small crater that formed after the top lobate scarp. The image is from a NY Times article which in turn used images from the July 4, 2008 issue of Science. The filled in impact crater at center left (with the second small white arrow and black arrow) is about 100 km across. See NASA/JPL’s Photojournal for more information about the wrinkle ridges inside the crater that formed when the lava plain flow solidified.

A special type of tectonic activity unique to Earth in our solar system is called “plate tectonics”. It is the major agent of changing the Earth’s surface and plays a key role in keeping the Earth habitable. It warrants its own section of discussion.

9.6.5 Plate Tectonics

The Earth’s lithosphere is broken up into chunks called plates with densities around 3. Oceanic plates are made of basalts (cooled volcanic rock made of silicon, oxygen, iron, aluminum, & magnesium). Oceanic crust is only about 6 kilometers thick. The continental plates are made of another volcanic type of silicates called granite. Continental crust is much thicker than oceanic crust—up to 35 kilometers thick. Continental plates are less dense than the oceanic plates. The mantle convection causes the crustal plates to slide next to or under each other, collide against each other, or separate from one another in a process called plate tectonics. Plate tectonics is the scientific theory that describes this process and how it explains the Earth’s surface geology. The Earth is the only planet among the terrestrial planets that has this tectonic activity. This is because plate tectonics probably requires liquid water to solidify the oceanic plates at the midocean ridges where seafloor spreading is happening (see below) and more importantly, the liquid water lubricates the aesthenosphere and softens the lithosphere enough so that the plates can slide past or under one another. Venus has enough interior heat to have convection in its mantle like the Earth, but through processes described in another section, Venus lost its water, so its plates are poorly lubricated at best.

Plate Tectonics Theory Evidence

Plate tectonics is a relatively recent theory having been proposed in the late 1960s and finally being verified enough so that it could be put in the introductory geology textbooks in the 1980s (remember all of the peer review, error-correction process that happens before something is fit to print in a textbook). What finally made scientists accept the theory? Today we can easily measure plate motion using GPS sensors on either side of plates. As in any robust scientific theory there are multiple lines of evidence supporting the theory. For plate tectonics, the evidence is continental motion, seafloor spreading, earthquake and volcano locations, and the difference between the seafloor crust and continental crust. I will give just a brief outline of the evidence for plate tectonics here. See the nice, detailed presentation from the USGS’s Dynamic Earth online text.

• Continental motion: The locations of rock-types and certain fossil plants and animals on present-day, widely separated continents would form definite patterns if the continents were once joined. For example the eastern side of South America fits nicely next to the western edge of Africa and several fossil areas match up nicely at those points of intersection.
• Seafloor spreading: An immense submarine mountain chain zig-zags beween the continents and winds its way around the globe. At or near the crest of the ridge, the rocks are very young, and become progressively older away from the ridge crest. The youngest rocks at the ridge crest always have present-day (normal) magnetic polarity. Stripes of rock parallel to the ridge crest alternate in magnetic polary (normal-reversed-normal, etc.)Alternating stripes of magnetically different rock are laid out in rows on either side of the mid-ocean ridges: one stripe with normal polarity and the adjoining stripe with reversed polarity. This happens when magnetite in molten rock at the ridge aligns itself with the Earth’s magnetic field. When the molten rock with the magnetite hardens, it “freezes” in the orientation of the Earth’s magnetic field at that time. The Earth’s magnetic field has changed polarity numerous times in its history with a 300,000 year average time interval between reversals (some reversals were just tens of thousands of years apart and others millions of years apart). When the Earth’s magnetic field changes polarity, newly rising molten rock at the ridges will have its magnetite aligned accordingly. New oceanic crust is forming continuously at the crest of the mid-ocean ridge and cools to become solid crust. The oceanic crust becomes increasingly older at increasing distance from the ridge crest with seafloor spreading. The result will be a zebra-like striping of the magnetic polarity in the rock that parallels the mid-ocean ridge. Further evidence for seafloor spreading comes from determining ages of the seafloor at various distances from the mid-ocean ridges.
• Earthquake and volcano locations: The locations of earthquakes and active volcanoes are not random but, instead, they are concentrated along areas thought to be plate boundaries—the oceanic trenches and spreading ridges.
• Seafloor crust vs. continental crust: From radioactive dating and crater-age dating, we know that the seafloor crust is less than 200 million years old while continental crust has a wide range of ages up to 4 billion years old. Also, the seafloor crust is thinner and denser than the continental crust. Earth is the only place in our solar system with two different types of crust.

Plate Tectonics Process

The figure below shows the boundaries of the major plates on top of a map of the Earth. The arrows show the direction of the plates with respect to each other. The white areas are elevations greater than 2400 meters (7900 feet) above sea level. This figure is an adaption of a map in the “Plate Tectonic Movement Visualizations” website of the Science Education Resource Center at Carleton College and the plate motion data from This Dynamic Earth of the USGS. Select the figure to bring up an enlarged version of it.

Places where warm rock from the asthenosphere rises along weak points in the lithosphere can push apart the lithosphere on both sides (see the figure below). These places are at the midocean ridges (such as the Mid-Atlantic Ridge that bisects the Atlantic Ocean) and continental rift zones (such as the East Africa Rift Zone). Sea-floor spreading caused the Atlantic Ocean to grow from a thin sliver 100 to 200 million years ago to its present size and now continues at a rate of about 25 kilometers per million years.

This pushing apart of some plates from each other means that others will collide. The oceanic lithosphere is cooled by contact with the ocean water. When oceanic crust runs into oceanic crust or into continental crust, the denser lithosphere material slides under the less dense lithosphere material, eventually melting in the deepest layers of the mantle. The region where the lithosphere pieces contact each other is called a subduction zone and a trench is formed there. At the subduction zone, the right combination of temperature, pressure and rock composition can create small pockets or fissures of molten rock in the solid asthenosphere that then rise up through cracks in the crust to create a range of volcanoes (see the figure below). In another section you will see that this has a profound effect on regulating the climate of the Earth.

When two continental pieces bump into each other, they are too light relative to the asthenosphere and too thick for one to be forced under the other. The plates are pushed together and buckle to form a mountain range. It also possible for two plates to slide past each other at what is called a transform fault such as the San Andreas Fault in California and the Anatolian Fault in Turkey.

Examples of ocean-continental plate subduction include the Juan de Fuca plate off the coast of northwestern United States subducting under the North American continental plate to create the Cascade volcano range, the Nazca plate subducting under the western edge of the South American plate to create the Andes range of volcanic mountains. An example of the ocean-ocean plate subduction are the chains of islands on the Asia side of the Pacific: the Aleutians, Japan, Philippines, Indonesia, and Marianas. An example of continent-continent plate collision is the Indian plate running into the Eurasian plate to create the Himalayas.

Conclusions

To end the Planet Interiors section, here is a summary of the terrestrial planet surface shaping agents at work today.

Surface Shaper Agent	Mercury	Venus	Earth	Moon	Mars
Impact Cratering	Yes	Minor	Minor	Yes	Yes
Volcanism (needs internal heat)	No (only long ago)	Yes	Yes	No (only long ago)	No (only in the past)
Tectonics (needs internal heat)	No (only long ago)	Yes	Yes	No	No (only in the past)
Erosion	No (no liquid or atmosphere)	No (no surface winds)	Yes (ice, water, air)	No (no liquid or atmosphere)	Yes (air today + water in past)

Looking at the table, we can draw some conclusions as to what planet properties will determine the type of planet surfacing that can occur. Impact cratering can occur on any object with a solid surface at any time. If a planet has an atmosphere and is still geologically active, then the effects of impact cratering will be erased. Volcanism and Tectonics require the planet to be of sufficient size to still have heat in its interior. Erosion requires an atmosphere with winds to work efficiently. Even better is if liquid can be present to add to the weathering by the atmosphere. Erosion works best on Earth. Earth is of sufficient size to hang on to its atmosphere (unlike the Moon). Earth is at a good distance from the Sun so it is not too hot for its atmosphere to either evaporate away or become excessively thick that winds will not blow on the surface (as is the situation with Venus). Also, its good distance from the Sun enables the surface temperatures to be warm enough for liquid water to flow (unlike Mars) and so its atmosphere does not freeze out on its surface as happens with Mars. Earth’s rotation is fast for its size (unlike Venus), so it can create complicated air circulation (wind) patterns as well as ocean currents. Also, rapid rotation enables the creation of the magnetic field shield to protect a planet’s atmosphere from the solar wind.

Vocabulary

• erosion
• impact cratering
• plate tectonics
• tectonics
• volcanism

Review Questions 5

1. Why are almost all impact craters round?
2. How can you use the number of craters to determine the age of a planet’s or moon’s surface?
3. The lunar highlands have about ten times more craters on a given area than do the maria. Does this mean that the highlands are ten times older? Explain your reasoning.
4. What determines if volcanism will make a steep-sided mountain or something with a gentler slope?
5. How do shield volcanoes compare in size to stratovolcanoes in diameter and height?
6. How do volcanic eruptions affect a planet’s atmosphere?
7. What does volcanism require as far as interior conditions?
8. How does erosion change the surface of a planet (or moon)?
9. What is the difference of a valley carved by glaciers vs. one carved by flowing water?
10. Besides wearing away geological features, what does the process of erosion do?
11. How is tectonics different than plate tectonics?
12. What does tectonics require as far as interior conditions?
13. How does the plate tectonics theory explain such things as the widening of the Atlantic Ocean, the Andes of South America and the Cascades of the northwestern U.S, and the high mountain ranges such as the Himalayas and the Rocky Mountains?
14. What is the evidence for plate tectonics?
15. What properties of a planet will determine what type of planet shaping can occur on it?
16. What type of planet shaping occurs on Mercury, on Venus, on Mars, and on the Earth and the Moon?

9.7 Earth-Venus-Mars

Venus, Earth, and Mars are approximately at the same distance from the Sun. This means they formed out of the same material and had approximately the same initial temperatures 4.6 billion years ago. Long ago these three planets probably had moderate enough temperatures suitable for life. However, Venus is now much too hot for life and Mars is too cold for life. What happened to these two planets and why are they so different from the comparative paradise here on Earth? This section explores these three planets in more detail in order to answer this important question and what it might say for the future of the Earth.

9.7.1 Venus

Venus’ cloudtops in UV (left) and Venus’ surface imaged with radar (right).Venus is about 95% the size of the Earth and has 82% of the Earth’s mass. Like the Earth, Venus has a rocky crust and iron-nickel core. But the similarities stop there. Venus has a thick atmosphere made of 96% carbon dioxide (CO₂), 3.5% nitrogen (N₂), and 0.5% other gases. Venus’ ever-present clouds are made of sulfuric acid droplets between 45 and 66 km above the surface. It is those clouds that continually block our view of Venus’ surface, so we must use radar imaging (bouncing radio waves off its hard surface) to “see through” the clouds. Between the equator and about 55 degrees latitude the lower clouds in Venus’ atmosphere move at about 210 km/h and the uppermost clouds move much faster at 370 km/h. Near the poles, the winds are weaker and do not change with height because of the huge hurricane-like vortex that exists there. At the center of the vortex, there are no winds. Close to the surface, the winds are also essentially non-existent.

At Venus’ surface, the air pressure is 92 times the Earth’s surface atmospheric pressure. Venus’ surface atmospheric pressure is the same as what you would feel if you were 1 kilometer below the ocean surface on the Earth. The deepest free-divers can get down to around 160 meters (and divers breathing special mixtures of gases can get down to 730 meters). If you want to send someone to Venus, that person would need to be in something like a diving bell.

Surface of Venus from the Venera 13 lander on March 3, 1982. Note the flat basaltic rocks that are still sharp and un-eroded. The spacecraft edge is at the bottom right corner of the image. The view extends out to the horizon at the top left corner of the image. The shiny piece on the middle right is the camera cover. Venera 13 lasted for 127 minutes before the extreme heat overcame the electronics.

Besides being in a diving bell, the Venus explorer would also need a very powerful cooling system: the surface temperature is 737 K (= 477° C)! This is hot enough to melt lead and is over twice as hot as it would be if Venus did not have an atmosphere. Why does Venus have such a thick atmosphere and why is it so hot on its surface? Venus is so hot because of a huge greenhouse effect that prevents heat from escaping to space. On Venus, the super-abundance of CO₂ in its atmosphere is responsible for the huge greenhouse effect. Why is Venus’ CO₂ all in its atmosphere while most of the Earth’s CO₂ is locked up in its sediments? Earth has some 35 to 50 entire Earth atmospheres worth of carbon dioxide in the form of carbonates. Venus’ greenhouse effect probably started from the presence of a lot of water vapor, but Venus is now a very dry place.

9.7.1.1 Runaway Greenhouse

Venus was originally cooler than what it is now and it had a greater abundance of water several billion years ago. Also, most of its carbon dioxide was locked up in the rocks. Through a process called a runaway greenhouse, Venus heated up to its present blistering hot level. Because Venus was slightly closer to the Sun than the Earth, its water never liquified and remained in the atmosphere to start the greenhouse heating. As Venus heated up, some of the carbon dioxide in the rocks was “baked out.” The increase of atmospheric carbon dioxide enhanced the greenhouse heating. That baked more carbon dioxide out of the rocks (as well as any water) and a runaway positive feedback loop process occurred. This positive feedback loop occurred several hundred million to a few billion years ago so Venus has been very hot for that length of time.

The loss of water from the rocks means that Venus’ rocks are harder than the rocks of Earth and its lithosphere is now probably too thick and hard and its aesthenosphere is too poorly lubricated for plate tectonics to occur. The water Venus originally had is now gone because of a process called dissociation.

9.7.1.2 Ultraviolet Dissociation of Water

Venus’ water was always in the gaseous form and could reach high enough in the atmosphere for ultraviolet light from the Sun to hit it. Ultraviolet light is energetic enough to break apart, or dissociate, water molecules into hydrogen and oxygen. The very light hydrogen atoms were able to escape into space and the heavier oxygen atoms combined with other atoms. Venus’ water was eventually zapped away. The Earth’s ozone layer prevents the same thing from happening to the water here.

9.7.1.3 Hydrogen/Deuterium Ratio

How is it known that Venus originally had more water? Clues come from comparing the relative abundances of hydrogen isotopes on Venus and Earth. An isotope of a given element will have the same number of protons in the atomic nucleus as another isotope of that element but not the same number of neutrons. An isotope with more particles in the atomic nucleus will be more massive (heavier) than one with less particles in the nucleus.

 

Ordinary hydrogen has only one proton in the nucleus, while the isotope deuterium has one proton + one neutron. Therefore, deuterium is about twice as heavy as ordinary hydrogen and will stay closer to the surface on average. Gases higher up in the atmosphere are more likely to escape to space than those close to the surface.

On Earth the ratio of ordinary hydrogen to deuterium (H/D) is 1000 to 1, while on Venus the proportion of deuterium is about ten times greater—the H/D ratio is 100 to 1. The H/D ratio on Venus and Earth are assumed to have been originally the same, so something caused the very light hydrogen isotopes on Venus to preferentially disappear. An easy explanation for it is the ultraviolet dissociation of water.

 

A summary flowchart of what happened on Venus is given in the Earth-Venus-Mars summary section. Water vapor started the greenhouse heating. Carbon dioxide was baked out of the rocks, further aggravating the greenhouse effect. A runaway greenhouse started. The end result was all of the carbon dioxide in the atmosphere and the water dissociated away. The flowchart in the Earth-Venus-Mars section up to the last arrow occurred several hundred million to a few billion years ago. The diamond at the end describes the current state: CO₂ maintains the extremely hot temperature.

Magellan radar image of three large craters in the northwestern portion of Lavinia Planitia that is in the southern hemisphere of Venus. Howe Crater in the foreground is 37.3 km in diameter. Danilova Crater to the upper left of Howe is 47.6 km in diameter. Aglaonice at the right is 62.7 km in diameter.

Gula Mons stands 3 kilometers above the rest of Eistla Regio. Lava flows extend for hundreds of kilometers across the fractured plains. The Venus Express spacecraft has seen evidence(reported April 2010) that some volcanoes at least were active very recently and perhaps still are.

 

There is a bit of uncertainty in how long ago the runaway process happened because of what happened to Venus’ entire surface about 750 million years ago. Venus has the same number of craters/area all over its surface and when you compare the number of large craters/area on Venus with other places in our solar system, you derive an age of 750 million years (give or take a few tens of millions of years) for all of the surface of Venus. It appears that Venus underwent a global repaving event involving a large amount of tectonics or volcanism or combination thereof. That activity would have vaporized any carbonates locked up in the rocks adding to any greenhouse effect already going on. The global repaving event would have also removed whatever water might have been in the mineral matrix of the rocks making the rocks much harder than before. This is one reason why the rocks around craters are much more jagged and sharp than they would be if they were Earth rocks (lack of erosion is another). The removal of the water from the rocks would prevented any further plate tectonic activity if there was any to begin with. Venus is much too hot and dry now for plate tectonics to work. Venus does have volcanism occurring today as a result of hot magma plumes reaching the surface at some of its shield volcanoes.

It may be that the loss of water led to the shutting down of plate tectonics and the steady build-up of heat in the interior because it could not be released like what happens with the Earth through plate tectonics. In this view, the global repaving would happen when the interior heat builds up to a critical point. Could Venus have been a much nicer place before the global repaving event, how much did the global repaving event contribute to the runaway process, and whether or not Venus has had multiple global repaving events are three questions for further research.

9.7.2 Mars

Views from the Viking Orbiters: centered on the Schiaparelli Crater (left) and centered on Valles Marineris (right).Mars is about half the diameter of the Earth and has 1/10th the Earth’s mass. Mars’ thin atmosphere (just 1/100th the Earth’s) does not trap much heat at all even though it is 95% carbon dioxide (CO₂). The other 3% is nitrogen (N₂). Because the atmosphere is so thin, the greenhouse effect is insignificant and Mars has rapid cooling between night and day. When night comes the temperature can drop by over 100 K (180° F)! The large temperature differences create strong winds. The strong winds whip up dust and within a few weeks time, they can make dust storms that cover the entire planet for a few months. Two “before-after” image sets are shown below. The first pair is from the Mars Global Surveyor of the Tharsis bulge side of the planet. The “before-after” images are about 1.5 months apart. The second pair is from the Hubble Space Telescope (HST) of the other side of the planet. The “before-after” images are about 2.5 months apart and the truly global dust storm was still going on. You can see the dust storm beginning in the HST image in the left image in the Hellas Basin at about the 4 o’clock position.

9.7.2.1 Liquid Water

Color view of river drainage system from Viking orbiter on left and black-and-white zoom-in of Nanedi Vallis canyon using Viking (wide-field view) and Mars Global Surveyor (magnified view) images on right.
In Eberswalde Crater is a “fossil delta” that shows that Mars experienced ongoing, persistent flow of water over an extended period of time. It also shows that some sedimentary rocks on Mars were deposited in a water environment. Select the image to find out more details leading to these deductions.	Fossil delta in Jezero Crater. Color-enhancing shows the clay-like minerals (green here) that were carried into the lake, forming the delta. The clays would be a good place to look for signs of ancient life.

What makes Mars so intriguing is that there is evidence for sustained running liquid water in its past. Some geologic features look very much like the river drainage systems on Earth and other features points to huge floods. The Mars Pathfinder studied martian rocks in the summer of 1997 and found some rocks are conglomerates (rocks made of pebbles cemented together in sand) that require flowing water to form. Abundant sand also points to widespread water long ago. More recently, the larger and more advanced Mars Exploration Rovers (one called “Spirit” and the other called “Opportunity”) have further strengthened the conclusion that there was liquid water on Mars in the past. Highly magnified images of the rocks examined by Opportunity (see image below) show a particular type of rippling patterns on the rocks that are formed under a gentle current of water instead of wind. In the image below the green stripes show the sedimentary layers laid down in flowing water and the blue lines show the boundaries between the layers. Furthermore, detailed chemical analysis of the compositions of the rocks by Opportunity show that they formed in mineral-rich water when the water got very concentrated with the minerals as the water evaporated (spectrum 1, spectrum 2, spectrum 3, spectrum 4, spectrum 5, spectrum 6 links).

More recently, Opportunity found veins of the mineral gypsum near the edge of the large crater Endeavour. The gypsum veins show that water flowed through the rocks and they are even stronger evidence for water than mentioned above.

The Mars Science Laboratory, “Curiosity”, landed in Gale Crater near the equator of Mars on August 5, 2012. It has a suite of instruments for identifying organic compounds such as proteins, amino acids, and other acids and bases that attach themselves to carbon backbones and are essential to life as we know it. It is also able to detect gases that could be the result of biological activity but it does not the instruments to determine if there is currently existing biological activity going on. It will identify the best possible sites of biological activity (past or present) for a follow-up mission to more definitely confirm. Gale Crater is 155 kilometers (96 miles) across with a large mountain (Mount Sharp) inside it that appears to be the remnant of an extensive series of deposits. The layers at the base of the mountain contain clays and sulfates that very likely formed in liquid water. Curiosity will also investigate: the geology of the area to figure out how the rocks and soil formed; how the atmosphere has changed through time and the cycling of water and carbon dioxide; and the radiation environment at the surface (photons and particles from the Sun and the rest of the galaxy). See the William M Thomas Planetarium’s MSL Landing page for more about Curiosity’s landing sequence, see the first pictures returned from Gale Crater, and details on why Gale Crater was chosen.

The image above shows sections of the first 360-deg color pan of Gale Crater where Curiosity landed. The image was brightened during processing. The crater wall is visible on the left and right sides of the image and Mount Sharp is near the center of the image with the dark dunes near its base containing clays and sulfates that very likely formed in liquid water probably 3.5 billion years ago. The several gray splotches in the foreground were produced by the descent stage sky crane’s rocket engines blasting the ground and blowing off the surface dust layer. Parts of the rover are visible as well. The black areas are places where the images making up the pan had not been all beamed to Earth yet.

At locations near where Curiosity landed in Gale Crater, it found deposits of gravel made of well-rounded pebbles eroded off of sedimentary conglomerate outcrops (left image). On Earth (right image of sedimentary conglomerate), rocks that are well-rounded are a common sign of rocks that have been transported by water in a river or stream. If the flow of water is great enough, the pebbles are lifted up in the flow or rolled along the bed and they get pounded against each other so the edges get rounded off. The sizes of the pebbles on Mars are too large to have been transported by wind; they must have been transported by a sustained, vigorous flow of water.

Features such as the gullies in the sides of craters provide a cautionary tale for how we have to be careful in assuming that water must be the cause of sinewy features (and how Mars can fool us if we are too Earth-like in our thinking!). The Mars Global Surveyor found places where gullies are etched into the sides of craters that themselves have very few smaller impacts inside of them. That means the crater walls are geologically young, so the gullies have to be even younger still. The orbiter also found gullies where bright new deposits were seen in images taken just four years apart from each other that seemed to be the result of water carrying sediments down the sides of the craters for a short time. However, the gullies formed mostly on the crater walls facing the poles. Also, the gullies were far more active (forming new features) in the southern hemisphere than in the north. These patterns better match the seasonal changes of carbon dioxide frost (dry ice) formation and thensublimation (when a solid turns directly to gas without the intermediate liquid phase). Mars’ southern hemisphere winters are longer and colder than those in the north, so more frost forms and piles up in the southern hemisphere. As the dry ice sublimates it causes the rock and soil to flow downhill. Such action by dry ice can happen on Mars but not the Earth because Mars can get extremely cold, colder than Antarctica.

Even with this cautionary tale, there does appear to be evidence for liquid water flowing relatively recently. In 2011 the team of scientists working with the Mars Reconnaissance Orbiter (MRO) released images of seven craters where thin (0.5 to 5 meters wide) dark streaks are seen to flow down steep slopes repeatedly during the summer seasons when temperatures on the surface there can reach -23º to +27ºC (-10º to +80ºF). In an effort to not bias our interpretation of them, the team has called the features “recurring slope lineae”. There are 12 to 20 other sites the team is keeping a close eye on to see if they too have the repeated dark flows.

Select image for other craters with time-lapse images from the MRO press release

Mars’ surface air pressure is much too low for pure liquid water to exist now. At very low pressure, water can exist as either frozen ice or as a gas but not in the intermediate liquid phase. If you have ever cooked food at high elevations using boiling water, you know that it takes longer because water boils at a lower temperature than at sea level. That is because the air pressure at high elevations is less. If you were several miles above the Earth’s surface, you would find that water would boil (turn into steam) at even room temperature! However, if the liquid is very salty water, then it may be able to exist long enough to flow partway down the crater walls before freezing or evaporating. The dark streaks that grow during the warm summer time could be the result of liquid brines near the surface of Mars breaking through to the surface. Because of the widespread presence of salts on the surface of Mars, even pure water from below would get mixed with the salts. Unfortunately, the spectrometer on board MRO does not have sufficient spatial resolution to analyze the very narrow dark streaks. To strengthen the argument for these being the result of salty water flowing downhill, research teams on Earth will simulate the conditions (soil composition, low air pressure, and temperature) in the lab to see what all could create the dark streaks. In 2012 the MRO team increased the number of recurring slope lineae to 15 and strengthened their conclusion that they are caused by briny water melting and seeping downhill through the soil (see also link).

Where there is liquid water, there is the possibility for life to arise. Tiny structures in a meteorite that was blasted from Mars in a huge impact of an asteroid look like they were formed by ancient simple lifeforms. However, there is still a lot of debate among scientists on that but strong evidence of contamination by terrestrial organic molecules has probably killed the possibility of conclusive proof of martian life in the meteorite. The dissenters are not wanting to be party poopers. They just want greater certainty that the tiny structures could not be formed by ordinary geologic processes. The great importance of discovering life on another world warrants great skeptism—“extraordinary claims require extraordinary evidence”. The search for martian life will need to be done with a sample-return mission or experiments done right on Mars.

The fact that Mars had sustained liquid water in the past tells us that the early Martian atmosphere was thicker and the surface was warmer from the greenhouse effect a few billion years ago. Some of the topics for follow-up research are: how long was there liquid water present on the surface; when did the liquid water disappear from the surface; how widespread was the liquid water; how much liquid water was there; and were there repeated episodes of liquid water appearing and then disappearing.

Life may have started there so current explorations of Mars are focussing on finding signs of ancient, long-dead life. An important step in that search is to determine how habitable Mars might have been long ago. Besides liquid water, life would need to have some source of energy to drive its metabolism. In early 2013, the Curiosity rover drilled into a sedimentary rock in Gale Crater. Drilling into the rocks may give us information about the conditions on Mars further back in time than what the Mars Exploration Rovers can give us with their wire brush tool.

The rock powder from Curiosity’s first drillings were gray with a hint of green, not red, from olivine and magnetite that have less oxygen than the hematite found in rocks studied by the Mars Exploration Rovers. The range of oxidation of the materials in the rock could be used as a sort of chemical battery by micro-organisms. In addition, the reduced oxygen minerals would be better at preserving organics than the more oxidized rocks studied by the Mars Exploration Rovers. The rock studied by Curiosity has sulfur (including sulfur dioxide and hydrogen sulfide), nitrogen, oxygen, phosphorus, and carbon (in the form of carbon dioxide)—key ingredients for life. The presence of calcium sulfate and other minerals suggest that the clay minerals were formed from neutral or mildly alkaline water that wasn’t too salty instead of the extremely high acidity and saltiness of the water that would have formed the rocks explored by the Mars Exploration Rovers. Examinations of other rocks in the Gale Crater area explored by Curiosity in late 2012-early 2013, show cracks between rocks filled with hydrated minerals that may indicate the Gale Crater floor was soaking wet more than once. At the time of writing, no organic compounds had been found yet by the SAM instrument on Curiosity, but one thing for sure is that ancient Mars was definitely habitable, able to support microbes long ago.

Any lifeforms living now would have to be living below the surface to prevent exposure from the harsh ultraviolet light of the Sun. Mars has no protective ozone layer, so all of the ultraviolet light reaching Mars can make it to the surface. The Viking landers that landed in 1976 conducted experiments looking for biological activity, past or present, in the soil but found the soil to be sterile with no organic matter (in the top several centimeters at least). The soil is more chemically reactive than terrestrial soil from the action of the harsh ultraviolet light. More recently, the Phoenix mission described below in the “Ice on Mars” section may have found another reason for the lack of organic matter in the soil: perchlorate in the soil would break down any organic compounds that would have been in the soil when the soil was heated up during the Viking experiments. In any case, Mars appears to have undergone significant global change. What changed Mars into the cold desert of today?

 

 

9.7.2.2 Atmosphere Escape

There are several ways for Mars to have lost its atmosphere:

1. Mars’ low gravity let the atmosphere leak away into space;
2. A lot of impacts of asteroids blasted part of the atmosphere away. Such large impacts occurred very frequently in the early solar system several billion years ago. The energy of the impacts could have been enough to push the gas away from a planet with small gravity.
3. Mars had a reverse greenhouse effect, called a runaway refrigerator, occur. Since Mars was slightly further from Sun than the Earth, Mars’ initial temperature was lower. This meant that the water vapor condensed to form a liquid water layer on the surface. Gaseous carbon dioxide dissolves in liquid water and can then be chemically combined with rocks. This would have happened on Mars long ago. The removal of some of the carbon dioxide caused a temperature drop from the reduced greenhouse effect. This caused more water vapor to condense, leading to more removal of atmospheric carbon dioxide and more cooling, etc. This positive feedback process is called a runaway refrigerator and is described in the first two panels on the left of the figure below. This runaway process occurred probably a billion years ago, so Mars has been cold for a long time. Mars’ water is now frozen in a permafrost layer below the surface and the atmosphere is very thin. Mars has undergone several dramatic climate swings, so it may have undergone a warming and cooling several times in its past with the latest cooling possibly being more recently than a billion years ago. During the warm periods some water may have flowed across Mars’ surface and then froze again when Mars returned to the very cold times. Mars is unfortunately too small to retain enough internal heat to drive something like plate tectonics. As explained in the Earth section, plate tectonics plays a key role in regulating a planet’s climate, so the planet avoids becoming either a hot Venus or a cold Mars.

   The runaway refrigerator is described in a flowchart on the Earth-Venus-Mars page. The flowchart up to the last dashed arrow occurred a LONG time ago. The box at the end describes the current state: frozen water and carbon dioxide below the surface and a very thin atmosphere.The runaway refrigerator theory recently received further support when the Mars Reconnaissance Orbiter found places where deposits of iron and calcium carbonates had been uncovered at large impact sites great distances apart from each other. These types of carbonates form most easily in the presence of large quantities of liquid water and fit the runaway refrigerator idea of atmospheric carbon dioxide dissolving in bodies of liquid water. The carbonate layers are buried under a few miles (about 5 kilometers) of younger rocks, including volcanic flows, similar to what Spirit found when exploring Gusev crater. At Gusev Crater, Spirit had to climb the hills near where it landed to find the older minerals that formed in the presence of liquid water sticking above the surrounding crater floor covered in lava flows (see “Panel 1” of the “Follow the Water” forum). Large impacts are able to uncover the deeper carbonate layers, so MRO will explore other large impact craters closely to see how widespread the buried carbonate layers are.

4. The atmosphere was slowly eaten away by the solar wind that is able to directly reach the upper atmosphere because Mars does not have a magnetic field. The fast-moving solar wind particles hit the upper atmosphere particles with such force to kick them to speeds faster than the escape velocity. See a video from the MAVEN site illustrating this process.
5. A combination of these effects. The NASA MAVEN mission scheduled to launch at the end of 2013 will investigate how and how fast Mars’ atmosphere is leaking away now and hopefully, provide the information we need to figure out what happened in the past. MAVEN is short for “Mars Atmosphere and Volatile Evolution Mission”.

Human explorers will need to use spacesuits on Mars’ surface. The low pressure would kill them in a fraction of a second without something to provide an inward pressure on their bodies. Explorers will also need to contend with temperatures that are way below the freezing point of water even during the day and have enough shielding to block the abundant ultraviolet light from the Sun.

9.7.2.3 Ice on Mars

One of the predictions of the runaway refrigerator is that there should be water ice below the surface. Mars does have polar ice caps made of frozen carbon dioxide (“dry ice”) and frozen water, but is there frozen water below the surface away from from the polar ice caps? Yes!

Yuty Crater is a type of crater called a rampart crater (or “splash crater”) because of the distinctive ridges along the edge of the “fluidized” ejecta. This image from the Viking 1 orbiter as well as others it took of craters in the surrounding area show features formed when frozen (or liquid?) water was melted and mixed with the dirt and rocks to flow like mud upon impact (see also the image of Yuty Crater from the Mars Global Surveyor and a high-resolution image of the fluidized ejecta flow or link 2).

The Mars Odyssey spacecraft orbiting Mars found that the highest concentrations of the sub-surface ice are near the poles from about latitudes 60 degrees and higher. The Phoenix Mars Lander that landed at the end of May 2008 scooped up ice and soil at its landing spot near the martian north pole south of the north polar cap. It uncovered water ice just a few centimeters below the surface.Phoenix also ran tests to see if the soil & ice could be habitable for microbial life. Phoenix did not have the capability to detect biological activity; it could only determine if life could exist in the soil at its landing spot. The answer: maybe. Phoenix found perchlorate that is toxic to most Earth life but it can be food for some types of microbes. Water vapor from the atmosphere could attach to the perchlorate to form a thin film layer of water for biological activity. In addition perchlorate in water can lower its freezing temperature enough to keep it liquid even in Mars’ cold temperatures.

“Holy cow!” image from the robotic arm camera of the Phoenix Mars Lander shows what looks like ice uncovered by the rocket exhaust upon landing.

Phoenix uncovered ice in this trench called “Dodo-Goldilocks” that sublimated (from ice directly to water vapor) over the course of four Mars days. The ice lumps seen in the lower left corner on day 20 are no longer there on day 24. If the ice was carbon dioxide, they would have sublimated away in less than a day at the temperatures of Phoenix’s location when the trench was dug.

 

The image above and others from the Mars Reconnaissance Orbiter show that frozen water is just below the surface at latitudes closer to the equator than thought possible before. The bright areas in the upper panels are are sub-surface water ice freshly exposed by meteorite impacts. They fade over 15 weeks as the water ice sublimates (bottom panels). See the MRO video archive for a nice video about this particular discovery (choose the September 24, 2009 video). These craters were near the location of the Viking 2 lander and the scientists figured out that if Viking 2 had been able to dig just 15 cm (6 in) deeper, it would have found the ice (33 years before Phoenix).

Some pieces of Mars are delivered to Earth as meteorites blasted from the surface of Mars from giant impacts. The ejecta from the blasts were moving fast enough to escape Mars’ gravity and eventually find their way to Earth. The Zagami meteorite, named after the place in Nigeria where it landed in 1962, is one example. A small slice of it is shown above. At least several dozen martian meteorites have been discovered so far. Trapped gases in them closely resemble the atmosphere analyzed by Viking, their distinctive compositions (very different from regular meteorites), and, in most cases, much younger ages than regular meteorites, tell us that these particular meteorites came from Mars. Some of them have carbonates in them which again tell us that liquid water was once present on Mars.

Go to Maps of all Mars images from orbit website

And just for fun: Zoom in to the “Face on Mars”. The “Face on Mars” is a remnant massif that attracted a lot of attention in the 1990s when a Viking 1 Orbiter picture of it, that made it appear like a face had been carved onto the martian surface, circulated around the web. Those with an over-active imagination thought the feature was artificial (made by ancient martian astronauts) and conspiracy theories were created of a NASA cover-up. The Viking image has poor resolution, poor lighting, and a number of “bit errors” that create black speckles, a couple of which were located at the feature’s “nostril” and a “dimple” on its chin. Later much higher resolution images from the Mars Global Surveyor and Mars Express spacecrafts show it to be a naturally-occuring geologic formation. The“`Face on Mars’ Zoom In” page shows where the “Face” is on Mars and increasingly higher-resolution views of the feature.

Finally, my notes from the January 13, 2011 update on the Mars Program.

9.7.3 Earth

Our home planet, the Earth, is the largest of the terrestrial planets with a diameter of 12,742 kilometers and a mass of 5.9736 × 10²⁴ kilograms. It has a moderately-thick atmosphere that is 78% nitrogen (N₂) and 21% oxygen (O₂). Although the atmosphere makes up less than 0.0001% of the Earth’s mass, it is a very important component. The Earth has the right surface temperature and atmospheric pressure for life and liquid water on the surface to exist. It is the only place that has either of these things. Some water is also in the form of water vapor and ice. The total amount of water on the Earth (in all phases) is about 0.023% of the Earth’s mass—the Earth is primarily rock and iron. The Earth is also a very beautiful place.

9.7.3.1 Free Oxygen

Compared to the other planets, the Earth has a bizarre atmosphere! The presence of free oxygen (O₂) is very unusual because oxygen loves to chemically react with other atoms and molecules. The oxygen in our atmosphere would soon disappear (within about 5000 years) if photosynthesizing organisms like plants and cyanobacteria (blue-green algae) did not regenerate the oxygen. In the process of photosynthesis, plants take in water, carbon dioxide, and sunlight and convert them to carbohydrates and oxygen. The oxygen is given off as a waste product and the carbohydrates are stored as a source of energy to be used later by the plants. Since life keeps oxygen in the form of O₂ and its fragile cousin, ozone (O₃) around, absorption lines of these two molecules from a planet beyond our solar system would be one signature of life on that planet. Ozone has a spectral signature in the infrared—the spectral band where a search for bio-markers would take place.

9.7.3.2 Liquid Water

Most of the Earth’s water is liquid and some is frozen. The rest that is water vapor works with carbon dioxide in the atmosphere to create a small greenhouse effect, raising temperature about 34° C. This natural greenhouse effect makes it warm enough on the surface for liquid water to exist. Besides making life possible, the liquid water also helps to keep the amount of atmospheric carbon dioxide from getting too high. Carbon dioxide dissolves in liquid water to form “carbonic acid” (soda water). Some of the dissolved carbon dioxide will combine with minerals in the water and settle to the ocean floor to form limestone. A similar process happens with the weathering process. Carbon dioxide dissolved in rainwater (and in snowmelt) combines with minerals eroded away from the mountains to carry carbonates down to the oceans. The amount of atmospheric carbon dioxide is also kept in check by biological processes.

9.7.3.3 Life’s Role in the Carbon Cycle

Plants extract atmospheric carbon dioxide in the photosynthesis process and use it to form organic compounds. Most of the carbon dioxide is released back into the atmosphere when plants decay (or are burned). Some organic material (plants and bacteria) is deposited in marine sediments. The organic material in the marine sediments may be converted to oil, natural gas, or coal if the temperature and pressure conditions underground are right. This locks up the carbon dioxide until these fossil fuels are extracted and burned. Burning them releases the carbon dioxide back into the atmosphere.

Aquatic plants extract carbon dioxide dissolved in the water to use in their photosynthesis process. Aquatic animals use the carbon dioxide and calcium in the water to make shells of calcium carbonate (CaCO₃). When the animals die, their shells settle to the ocean floor where, after years of compacting and cementing, they form limestone, locking up the carbon dioxide. A great majority of the Earth’s carbon dioxide is buried deep below the surface in the form of carbonates. Some of the locked up carbon dioxide is released into the atmosphere via geologic heating processes such as volcanism.

9.7.3.4 Plate Tectonics Role in the Carbon Cycle

Earth is unique in that its crust is broken up into chunks called “plates” and these “plates” jostle about because of the convection motion of the mantle below the crust. Among other things plate tectonics does in keeping our planet habitable, plate tectonics play a key role in regulating the amount of carbon dioxide in the Earth’s atmosphere. Carbon dioxide in the atmosphere dissolves in rainwater. The slightly acidic rainwater erodes land rocks and the broken down minerals are carried to the oceans via the runoff. Calcium in the broken down minerals combines with the dissolved carbon dioxide in the oceans to create carbonates such as limestone at the ocean floor. Through plate tectonics, mountains are formed when plates collide and weathering of the mountains removes carbon dioxide from the atmosphere and puts it onto the ocean floor. Through plate tectonics, the limestone and other carbonate minerals are carried to subduction zones where they are melted. The melted rock releases carbon dioxide through volcanoes. (Plate tectonics giveth; plate tectonics taketh away.) Volcanoes are the major natural (non-human) way that carbon dioxide is released back into the atmosphere over tens of millions of years of time (in the short term they are just a hundredth of the human annual contribution). The figure below shows some of the plate tectonics events with climate changes over the past 540 million years using the changing ratio of oxygen-18 to oxygen-16 as a proxy for temperature changes as discussed in a previous section. A one part in 1000 change in the oxygen-18 corresponds to a 1.5 to 2 degrees Celsius change in the sea surface temperature.

The formation of the limestone happens most easily in shallow water. On Earth the presence of continents makes it possible for places of shallow water to exist. The continents are created from lower-density material than oceanic plates through the subduction process of plate tectonics. Liquid water plays a key role in helping the subduction process of plate tectonics to work by lowering the melting point of the oceanic crust and keeping the lithosphere pliable enough to bend and descend far enough into the mantle to melt. In turn the temperature regulation of plate tectonics enables liquid water to remain on the Earth’s surface.

The temperature regulation happens because of a negative feedback process that cools the Earth if it gets too hot and warms the Earth if it gets too cool. The rate that carbon dioxide is removed from the atmosphere depends on the temperature such that the higher the temperature, the higher the rate that carbon dioxide is removed. If the Earth warms up, there will be more evaporation and rainfall, resulting in greater removal of atmospheric carbon dioxide. The reduced atmospheric carbon dioxide leads to a weakened greenhouse effect that counteracts the initial warming and cools the Earth back down. If the Earth cools off, the rainfall decreases, resulting in less removal of atmospheric carbon dioxide. The atmospheric carbon dioxide level will build back up because of the outgassing of volcanoes. A strengthened greenhouse effect counteracts the initial cooling and heats the Earth back up. This “thermostat” temperature regulation takes at least half a million years to adjust the temperature so we cannot look to it to solve the short-term changes to the global climate discussed in the next sub-section.

The thermostat did not work as well before about 540 million years ago. The Snowball Earth episodes discussed in a previous section alternated with episodes of extreme warmth (“hothouse Earths”) with large amounts of carbon dioxide in the atmosphere. The large amount of carbon dioxide would have been needed to provide the amount of greenhouse warming necessary to melt the ice. Recall that glaciers leave piles of rubble. Several layers of rubble piles are found in dozens of places on the Earth dated between about 800 million to 600 million years ago and these places would have been near the equator at those times. Right above the glacial rubble pile layers are found layers of carbonate deposits (“cap carbonates”). Ordinarily, such a juxtaposition of these two types of layers is not found, nor expected because glacial deposits are usually found in higher (cooler) latitudes while carbonates form easily in lower (warmer) latitudes. The Snowball Earth theory predicts that a hothouse Earth would follow a snowball Earth; just what is seen in the geological record. The carbon cycle would then have worked to bring the temperatures to more moderate levels. Interestingly, it may be the presence of multi-cellular life, particularly worms, that has prevented the large temperature swings from happening again. Worms in the ocean sediments wiggling about prevent the methane and carbon dioxide from getting locked away and bringing on too large a drop in the greenhouse effect. Complex, multi-cellular life did not evolve until around 540 million years ago—after the last Snowball Earth-hothouse Earth swing.

Sections Review

Vocabulary

• carbon cycle (carbon dioxide cycle)
• dissociation
• photosynthesis
• runaway greenhouse
• runaway refrigerator

Review Questions 6

1. What distinguishes Venus from the rest of the planets? Why is Venus so hot?
2. What is a runaway greenhouse?
3. How does the ultraviolet-water interaction explain why Venus is so dry? How is the same process prevented on the Earth?
4. What major geological event occurred on Venus over half a billion years ago? How do we know?
5. If Mars’ atmosphere is over 90% carbon dioxide like Venus’, why does it have such a small greenhouse effect?
6. What atmospheric phenomenon can quickly wipe out any view of Mars’ surface from above? What causes this phenomenon?
7. Why does Mars have such a thin atmosphere? What is the runaway refrigerator?
8. How does liquid water remove carbon dioxide gas from an atmosphere?
9. Why does Titan, a moon of Saturn less massive than Mars, have a more extensive atmosphere than Mars and Earth? (Recall the factors that affect atmosphere thickness.)
10. How do we know that liquid water once flowed on the surface of Mars?
11. If life exists on Mars today, where would it be found and why?
12. Where is water ice found today on Mars and how do we know? How deeply is it buried?
13. What distinguishes Earth from the rest of the planets? What is so unusual about its atmosphere and what produces this unusual feature?
14. What are the different ways that life removes carbon dioxide from the Earth’s atmosphere?
15. Where do coal, oil, and natural gas come from? What happens when they are burned?
16. What is a natural (non-human) way that most of the carbon dioxide is returned to the atmosphere on Earth?
17. How does plate tectonics regulate the climate of the Earth?
18. What is the interaction of plate tectonics and liquid water?
19. What are the benefits of the presence of the ozone layer?

9.8 Moons and Rings

This section gives a brief discussion of the large moons in the solar system and the characteristics of the rings found around all of the Jovian planets. The best known moon is the only other object that humans have explored directly—the Earth’s moon. I will discuss the Moon first and then move to the large moons of the Jovian planets. The two moons of Mars and most of the moons of the Jovian planets are small, rocky objects about the size of a large city or smaller. Most of them are probably asteroids that wandered too closely to the planet and got trapped by the planet’s gravity.

9.8.1 The Earth’s Moon

The Moon is about one-quarter the diameter of the Earth—if placed on the United States, it would extend from Los Angeles to almost Washington D.C. The Moon has held a special place in history. This is because it moves quickly among the stars and it changes—it goes through a cycle of phases, like a cycle of birth, death, and rebirth (see the Moon motions document). The Moon is also primarily responsible for the tides you experience if you spend any time near the coast (see the tides section of the gravity chapter). When Galileo looked through his telescope, he discovered a wondrous place. The Moon became a place to explore. Galileo discovered impact craters, mountains and valleys. The Moon is rough like the Earth.

The Moon without a telescope.

The Moon with a telescope (from Lick Observatory; original image no longer available online)

The Moon also has large, dark smooth areas covering about 17% of the Moon’s surface that people originally thought were seas of liquid water so they are called mare (Latin for “sea”—they are what make out the face on the Moon). Now it known that the mare are vast lava flows that spread out over many hundreds of square miles, covering up many craters that were originally there. The mare material is basaltic like the dark material on the Earth’s ocean crust and that coming out many of our shield volcanoes (e.g., the Hawaiian islands). Mercury also has maria but they are lighter in color because of the different chemical composition and they do not stand out from its heavily cratered areas.

Liquid water cannot exist on the Moon because of the lack of an atmosphere—the Moon has only 1/81 the Earth’s mass and about 1/6th the Earth’s surface gravity. If there is any water to be found on the Moon, it will be in a frozen state in a place of constant shade such as deep craters near the poles. Recent missions have discovered some of those ice blocks near the poles (Clementine, Lunar Prospector, LCROSS). The ice blocks will be the source of water for any humans that decide to set up bases on the Moon.

9.8.1.1 Craters

The Moon’s surface is almost as old as the Earth. The rough highland regions are 3.8 to 4.2 billion years old and the younger maria are between 3.1 and 3.8 billion years old. All of the planets and moons experienced a period of heavy bombardment about 3.8 billion years ago that lasted for about 500 million years as most of the remaining chunks of rock left over from the formation of the solar system pelted the planet and moon surfaces. The Moon, some of the moons of the giant planets, and Mercury preserve a record of this bombardment. The record of this heavy cratering was erased on the Earth long ago because of erosion and geologic activity that continues to this day. The Moon has no erosion because of the lack of liquid water and an atmosphere and it is small enough that its interior cooled off long ago so geologic activity has essentially ceased (an occasional very small moonquake can still occur). The small size of the Moon meant that not much heat could be stored from its formation and its small size also means that any remaining heat can easily escape to space (the ratio of its volume to its surface area is smaller than that for the Earth).

9.8.1.2 Interior and Composition

The average thickness of the Moon’s crust is between 34 and 43 kilometers thick. The strong tides from the Earth pulled the early Moon’s liquid interior toward the Earth, so the far side’s crust is thicker than the near side’s crust. The thinness of the near side’s crust is also why there are more mare on the near side than the far side. The near side was thin enough to be cracked apart when large asteroids hit the surface and formed the mare but the far side crust was too thick.

Our knowledge about the Moon took a huge leap forward during the Apollo missions. One main science reason for going to the Moon was to return rock samples to find about their ages and composition. Using their knowledge of geology gained from the study of Earth rocks, scientists were able to put together a history for the Moon. The Apollo astronauts also left seismometers on the Moon to detect moonquakes that can be used to probe the interior using seismology.

The Moon’s density is fairly uniform throughout and is only about 3.3 times the density of water. A recent re-analysis of the Apollo seismic data shows that the Moon has a small iron-rich core made of three parts: a solid inner iron core 480 kilometers in diameter, a fluid iron outer core shell 90 kilometers thick, and a third part unique to the Moon that is a shell of partial melt about 150 kilometers thick (see also ASU link). The iron core also contains a small percentage of lighter elements such as sulfur. Further refinements of our view of the Moon’s interior will undoubtedly occur as scientists analyze all of the gravity data from the recently-completed GRAIL mission.

 

(Red dots are locations of moonquakes that were measured by the Apollo seismometers on the surface.)The small core is a sharp contrast from planets like Mercury and the Earth that have large iron-nickel cores and overall densities more than 5 times the density of water. The Moon’s mantle is made of silicate materials, like the Earth’s mantle, and makes up about 90% of the Moon’s volume. The temperatures do increase closer to the center and are high enough to partially liquify the material close to the center. Its lack of a large liquid iron-nickel core and slow rotation is why the Moon has no magnetic field.

Lunar samples brought back by the Apollo astronauts show that compared to the Earth, the Moon is deficient in iron and nickel and volatiles (elements and compounds that turn into gas at relatively low temperatures) such as water and lead. The Moon is richer in elements and compounds that vaporize at very high temperatures. The Moon’s material is like the Earth’s mantle material but was heated to very high temperatures so that the volatiles escaped to space.

9.8.1.3 Formation

There have been a variety of scenarios proposed to explain the differences between the Moon and Earth. The one that has gained acceptance after much study is the giant impact theory developed by Hartmann and Davis in the mid-1970s. Earlier theories came in a variety of flavors.

1. The capture (pick up) theory proposed that the Moon formed elsewhere in the solar system and was later captured in a close encounter with the Earth. The theory cannot explain why the ratios of the oxygen isotopes (Oxygen-16 vs. Oxygen-18) is the same as that on the Earth but every other solar system object has different oxygen isotope ratios. The theory also requires the presence of a third large body in just the right place and time to carry away the extra orbital motion energy.
2. The double planet (sister) theory said that the Moon formed in the same place as the Earth but it could not explain the composition differences between the Earth and Moon.
3. The spin (daughter or fission) theory said that the Earth rotated so rapidly that some of its mantle flew off to the form the Moon. However, it could not explain the composition differences. Also, the spun-off mantle material would more likely make a ring, not a moon and it is very unlikely that the Earth spun that fast.

The giant impact theory proposes that 50 million years after the Earth formed, a large Mars-sized object hit the Earth and blew mantle material outward, some of which later recoalesced to form the Moon. Most of the mantle material from the Earth and the giant impactor combined to make a larger Earth. The Earth had already differentiated by the time of the giant impact so its mantle was already iron-poor. The impact and exposure to space got rid of the volatiles in the ejecta mantle material. Such an impact was rare so is was not likely to have also occurred on the other terrestrial planets. The just-formed Moon was only about 64,000 kilometers (40,000 miles) from the Earth and it has been spiraling away from us ever since (though, at an ever-decreasing rate). The tides the Earth experienced from the Moon at that early time was enough to raise solid ground 60 meters during high tide and the Earth was spinning much faster back then—one day was about five hours long.

The one “drawback” of the theory is that it has a lot of parameters (impactor size, speed, angle, composition, etc.) that can be tweaked to get the right result. A complex model can usually be adjusted to fit the data even if it is not the correct one (recall Ptolemy’s numerous epicycles). But the giant impact theory is the only one proposed that can explain the compositional and structural characteristics of the Moon.

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The images above are from paintings by one of the authors of the giant impact theory, William Hartmann. The first is at the time of impact and the second is five hours after the impact based on computer reconstructions of the impact. See Hartmann’s online painting catalog for many more space art paintings.

9.8.2 Jupiter’s Large Moons

Jupiter holds a special place in history because of the discovery by Galileo in 1610 of four large moons orbiting it. This observation gave Galileo strong evidence against the popular Earth-centered universe of his day. Like the Earth, Jupiter was a planet with moons orbiting it. Galileo took the Jupiter system to be like a miniature solar system. These four satellites are called the Galilean satellitesin honor of their discoverer. In order of increasing distance from Jupiter they are Io, Europa, Ganymede and Callisto. Since then, fifty-nine other moons have been discovered orbiting Jupiter. The Galilean satellites are of particular interest here.

The Galilean satellites (Jupiter’s four largest moons) to the same scale. Our Moon is also shown for reference.

Cutaway views of the possible internal structures of the Galilean satellites. Clockwise from top left: Io, Europa, Callisto, Ganymede. See the text below for information about the internal structure.

9.8.2.1 Io

The Galilean moon closest to Jupiter is Io. Io has no impact craters even though it has a rocky, solid surface. The surface must be very young because something has erased the impact craters. Even though Io has nearly the same size (3643 km across) and density (3.53X water) as the Moon, Io is the most geologically active world in the solar system. Io has many volcanoes and all of its craters are volcanic in origin. It is so active despite its small size because of the enormous stresses it experiences from Jupiter.

Even though Io is about the same distance from Jupiter as the Moon is from the Earth, Io experiences much stronger tidal stretching because Jupiter is over 300 times more massive than the Earth—Io’s rock surface bulges up and down by as much as 100 meters! Io also takes 1.77 days to orbit Jupiter—compare that with the 27.3 days that the Moon takes to orbit the Earth. Io’s orbit is kept from being exactly circular by the gravity of its Galilean neighbor Europa and the more distant Ganymede. Io, Europa, and Ganymede have a 4:2:1 orbital resonance that keeps their orbits elliptical. For every four orbits of Io, Europa orbits twice and Ganymede orbits once. Io cannot keep one side exactly facing Jupiter and with the varying strengths of the tides because of its elliptical orbit, Io is stretched and twisted over short time periods. The tidal flexing heats Io’s interior to the melting point just as kneading dough warms it up. The heat escapes through powerful eruptions spewing sulfur compounds in giant umbrella-shaped plumes up to almost 300 kilometers above the surface. The tidal heating from Jupiter has driven away much of the volatile materials like water, carbon dioxide, etc. Io’s surface is a splotchy mixture of orange, yellow, black, red, and white. The colors are created by sulfur and sulfur dioxide at various temperatures in liquid and solid states.

A note of caution before proceeding further: although heating of the interior by tidal effects is a significant reason why some of the moons of the jovian planets exhibit geologic activity, it is not the only reason. Tidal heating cannot explain all or any of the activity seen on some of the icy moons. Other mechanisms such as rotational shearing from a wobbly rotation axis can play a role. Ultimately, it is the composition of the icy moons that makes the difference. The ices are able to deform and melt at lower temperatures than the silicate and metal rocks found in the inner terrestrial planets and their moons.

9.8.2.2 Europa

The next moon out from Jupiter is the smooth, white moon called Europa. It is smaller than Io (3122 km across) and has a density of 3.01X water. Europa is of particular interest to astronomers because of what is below its ice crust. Europa has a rocky core surrounded by a deep ocean of liquid water, 100 kilometers deep, that is frozen on the surface. The Galileo spacecraft provided high-resolution images of its surface and showed giant blocks of ice that appear to have been broken off and floated away into new positions—see the image below. Below is a false-color image to highlight the differences in the surface. Reddish-brown areas represent non-ice material resulting from geologic activity. Dark blue areas are coarse-grained ice plains and light blue are fine-grained ice plains. The long, dark lines are ridges and fractures some of which extend more than 3000 kilometers that result from tectonic activity.

Europa has no impact craters which means the surface is very young. Depending on the assumptions of whether it is asteroids or comets pelting the surface, Europa’s surface might be as young as 100 million years old if asteroids make the craters or still have activity today if comets make the craters. The Galileo spacecraft found that Jupiter’s magnetic field at Europa changes in strength as Jupiter spins. A changing magnetic field can produce electrical currents in a salty ocean that will in turn produce a magnetic field to counter Jupiter’s magnetic field (something called Lenz’s Law). The presence of an induced magnetic field at Europa is a strong argument for the presence of a salty global ocean.

Europa is farther from Jupiter than Io, so the tides it feels from Jupiter’s enormous gravity are less. It takes about 3.5 days to go around Jupiter in its elliptical orbit. Weaker tides over a longer time period mean the tidal flexing is less than what Io experiences, but the calculated amount of tidal heating at Europa could be enough to maintain liquid water. Europa is too small for radioactive decay in its rocky core to provide enough heating. Geological activity on the moons of the jovian planets, even those smaller than Europa, is possible not only because of tidal heating but also because of their composition. Ice melts and flexes at lower temperatures than rock. The flexing of the ice from changes in the tides, as it moves around Jupiter in its elliptical orbit, creates an impressive system of cracks on the surface. The cause of the dark colors of the cracks is unknown, but it may be due to organic materials or salts.

Careful observations of how the Galileo spacecraft moved in Europa’s gravity field enabled scientists to determine the ocean thickness. The ocean of liquid water below Europa’s icy surface may extend down several tens of kilometers (or more). More importantly, the pressure at the ocean bottom could still be small enough that the liquid water could be contact with the rocky mantle that could supply nutrients for life. Could life forms have developed in the warm waters below the icy surface?

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Recent discoveries of fish, albino crabs, and 10-foot-long tube worms huddled around active volcanic vents on the Earth’s ocean floor far below where the sunlight energy can penetrate has bolstered the view that Europa could harbor life below its icy surface away from sunlight. Before the discovery of life around the geothermal vents, scientists thought that all life depended on sunlight. More recently, bacteria on Earth have been found to exist in rock a few kilometers below the sea floor and land surface. Clearly, life is more versatile than originally thought. Europa is the most likely place in our solar system for current life to exist beyond the Earth, even more likely than Mars. The proposed Europa Jupiter System mission would derive the thickness of the ice shell from how much Europa bulges with the changing tides as it orbits Jupiter. A thicker ice shell is stiffer and will not bulge as much as a thin ice shell. See the EJSM video for a visualization of how this would work and the Solar System Exploration EJSM website for other details. NASA would send a spacecraft to Europa and ESA would send a spacecraft to Ganymede, another moon thought to have an ocean below its surface. Further investigation of the Galileo data shows that there may be large shallow lakes within the top ice layer that could provide a way of exchanging material between the surface and the ocean beneath. That would make Europa more habitable even if the top ice shell is thick.

9.8.2.3 Ganymede and Callisto

The largest of the Galilean satellites (and the largest moon in the solar system) is Ganymede. At 5262 km across, Ganymede is larger than Mercury but because Ganymede orbits a planet, it is classified as a moon. Ganymede orbits Jupiter in 7.15 days. Its density of 1.94X water shows that it is made of half rock and half water ice. Its water ice composition enabled it to be more geologically active than Mercury—ice melts and flexes at a lower temperature than rock and metals of the inner terrestrial planets (and the Moon). Ganymede has bright grooved areas with few craters right next to much older dark areas with more craters. The bright, parallel ridges may have been caused by a plate tectonic process that was short-lived. Water may have gushed forth or ice squeezed up between the plate margins. Craters on the parallel ridges and older dark areas indicate that Ganymede’s geological activity probably stopped hundreds of millions of years ago.

The Galileo spacecraft detected a magnetic field generated by Ganymede itself as well as an induced one from the varying Jupiter magnetic field strength at Ganymede. The internally generated magnetic field is probably generated by convection in a liquid iron core heated by radioactive decay and tidal heating. Ganymede’s orbit is kept slightly eccentric because of the 4:2:1 orbit resonance it shares with Europa and Io so the tides do vary in strength. The induced magnetic field is evidence for a liquid water layer. At great enough pressures, water will turn solid. Ganymede’s water layer is hundreds of kilometers thick, so the liquid water part would be a thin layer sandwiched between a regular ice sheet above and a high density ice sheet below. That would mean that the liquid water layer would be cut off from any nutrients from the rocky mantle.

The second largest of the Galilean satellites (4821 km across) and the farthest from Jupiter is the heavily-cratered moon called Callisto. Callisto orbits Jupiter in 16.7 days. It has a density of 1.83Xwater, so it has proportionally more frozen water surrounding a smaller rocky core than Ganymede. Callisto’s surface does not appear to have undergone any sort of geological activity because of the lack of tidal heating. Its interior is partially differentiated—ice layer on top and a rock and ice mixed together core. Callisto has a huge impact site called Valhalla that was produced about 4 billion years ago. When the asteroid hit Callisto, it exploded on impact. The explosion heated the ice to above the melting point and the shock waves produced a ripple pattern away from the impact site. The ripples later froze so Valhalla now looks like a big “bull’s eye”.

Callisto has an induced magnetic field, so it may have a salty liquid water ocean layer beneath its ice crust. The side opposite Valhalla shows no grooves or hilly terrain as one would expect for a stiffer material (as for example, what we see on the opposite side of Mercury from its giant Caloris impact basin). A liquid water layer might explain how the shock waves from the Valhalla got dampened out by the time they reached the opposite side of Callisto.

(left) Boundary between an ancient, dark terrain (bottom) and younger, brighter fracture zones (top) on Ganymede as seen by Galileo. (right) The giant impact feature on Callisto called Valhalla is just above center right of this image taken by Voyager. The image has been enhanced to highlight the ripple pattern from the impact. Images courtesy of NASA/JPL

9.8.3 Titan

Saturn’s largest satellite (of 60 orbiting Saturn) is the mysterious world called Titan. It has slightly greater diameter (5150 km), density, and mass than Callisto. At 1.22 million kilometers from Saturn, it takes 15.9 days to orbit Saturn. With a density of 1.881X water, Titan is probably half rock, half ice. Careful observations of how the Cassini spacecraft moves in Titan’s gravity field have shown that Titan’s interior is only partially differentiated (like Callisto). Below the frozen surface may be an internal ocean of liquid water (or water-ammonia mixture) sandwiched between two thick ice layers surrounding a rock-ice mixture core.

What is special about Titan is that it has a thick atmosphere with a surface air pressure about 1.5 times thicker than the Earth’s. Even though Titan’s mass is even smaller than Mars’, it is so cold (just 95 Kelvin) that it has been able to hold on to its primordial atmosphere. The atmosphere is made of cold molecular nitrogen (95%) and methane (about 5%). Other organic molecules have been detected in its atmosphere. They are formed from solar ultraviolet light and high energy particles accelerated by Saturn’s magnetic field interacting with the atmospheric nitrogen and methane. The molecules of nitrogen and methane are split apart (photodissociation) and the atoms recombine to make a thick haze layer of mostly ethane that blocks our view of the surface in visible light. When the droplets of the organic molecules get large enough, they rain down to the surface as very dark deposits of liquid methane and ethane. Methane bubbling up from below the surface is thought to replenish the methane lost in its atmosphere from photodissociation. The picture of Titan, Triton and the Moon at the end of this sub-section shows hazy Titan as viewed in visible wavelengths from the Voyager spacecraft. Unfortunately, Voyager’s cameras were precisely tuned to the wrong wavelengths so it could not peer through the haze layer. Therefore, all it saw was an orange fuzz ball.

Titan’s brew of organic compounds is probably like the early Earth’s chemistry. Its very cold temperatures may then have preserved a record of what the early Earth was like before life formed. This possibility and the possibility of lakes or oceans of methane and ethane hidden under a haze of organic compounds made Titan the special subject of a Saturn orbiter mission to follow-up the Voyager fly-by mission. The Cassini spacecraft is now orbiting Saturn and flying by its numerous moons as part of its mission with special attention focussed on Titan. Using infrared wavelengths and radar, Cassini has been able to peer through the hazy atmosphere. The picture below is a mosaic of 16 images taken at infrared wavelengths coming from the surface and that pass through the atmosphere easily to Cassini’s camera.

Cassini managed to sample particles from the uppermost levels of Titan’s atmosphere (many hundreds of kilometers above the surface) and found that there were traces of oxygen in Titan’s upper atmosphere, probably from the photodissociation of water escaping Enceladus (see below) into hydrogen and oxygen. The presence of trace amounts of oxygen enables a greater variety of chemical compounds to be made with the energy of sunlight than just nitrogen and methane alone. Scientists simulating the conditions of Titan’s upper atmosphere by mixing together simple compounds together under very low densities and bathing them with various wavelength bands of light have been able to create complex organic compounds, and an experiment reported in October 2010 (see also the LPL Spotlight article or the UA news release) produced all five of the nucleotide bases of life (adenine, cytosine, uracil, thymine, and guanine) and two amino acids (glycine and alanine) when a mixture of molecular nitrogen, methane, and carbon monoxide were subjected to microwaves. Early Earth with only trace amounts of oxygen in its atmosphere might have produced the first nucleotides and amino acids in the same way.

Another probe called Huygens, built by the European Space Agency, hitched a ride on Cassini and parachuted down to Titan’s surface in January 2005. The color picture below (left) is Huygen’s view from the surface of Titan. The probe settled 10 to 15 centimeters into the surface. The various landscapes of Titan look surprisingly familiar—like landscapes here on Earth. The mechanics of Titan’s hydrogeological cycle is similar to the Earth’s but the chemistry is different: instead of liquid water, Titan has liquid methane and instead of silicate rocks, Titan’s rocks are dirty water ice. Liquid methane below the surface is released to the atmosphere to replenish that lost to the formation of the photochemical smog that eventually gets deposited in the soil. Methane rain washes the higher elevations of the dark material and it gets concentrated down in valleys to highlight the river drainage channels (see picture below right). Later images from Cassini have revealed huge methane and ethane lakes that change shape, presumably from rainfall of liquid hydrocarbons (see also). More recent images show methane rain falling in the equatorial regions of Titan as spring unfolded in the Saturn system in late 2010. Besides erosion, Titan may have signs of tectonic activity (and see also) and volcanism (and see also). There are, of course, some impact craters but fewer than 100 have been seen—small bodies burn up in Titan’s atmosphere and erosion, tectonics, and volcanism erase others. Its icy composition and its eccentric orbit might mean that tidal heating added to radioactive decay are enough to provide the internal heat (recall that water ice melts and flexes at lower temperatures than the rocks of the inner planets).

The montage below includes a radar map of the lakes near the north pole of Titan. They are filled with liquid ethane and methane and are fed from sub-surface seepage and rainfall. Looking a lot like lakes on the Earth, you can see bays, islands, and tributary networks. The large lake at the top of the radar map is larger than Lake Superior on Earth. Kraken Mare, of which a small portion of is visible in the lower left part of the map, is as big as the Caspian Sea on the Earth. There are also lakes near the south pole of Titan. [Data used to create the montage: 939 nm image, 5 micron glint image, andradar image.]

Titan, Triton, and our Moon to the same scale.

9.8.4 Enceladus

Enceladus is the fourth largest moon of Saturn at 504 km in diameter. It is shown in front of the much larger Titan in the image at left from Cassini. Enceladus orbits 238,000 kilometers from Saturn in 1.37 days. Despite its small size, Enceladus is a moon of large interest because it has the highest albedo of any major moon (1.0) and it is geologically active. Tidal heating supplies only a small amount (about 1/5th) of the internal heat for this moon. Recent simulations show that if Enceladus has a slight wobble in its rotation of between 0.75 and 2 degrees, the wobbling could generate about five times more heat than tidal heating as well as produce it at the observed locations of greatest heat in the fissures in its southern hemisphere. Geological activity is helped by Enceladus being mostly ice—its density is 1.61X water. Recall that ices can deform and melt at lower temperatures than silicate and metal rocks.

Enceladus has geysers spurting water (vapor and ice) from its south pole that point to a large ocean of liquid water below its icy, mirror-like surface. The geysers can be seen when one is on the other side of Enceladus looking back toward the Sun. The small particles scatter the sunlight forward toward the viewer. Geyser material is able to escape Enceladus and become part of the E-ring of Saturn. Enceladus’ activity appears to be localized to the southern hemisphere. Its northern hemisphere has many more craters. Recent sampling of the geyser material has found salts (sodium chloride and potassium chloride) and carbonates mixed in with the water. That means the liquid water layer is in contact with the rocky core instead of being sandwiched between ice layers. If there is an ocean below the icy surface, should Enceladus be another place to look for life besides Europa?

In this image above taken in November 2009, more than 30 individual jets shoot water vapor and ice up hundreds of kilometers from the south pole region.

The south pole region of Enceladus is a stark contrast from regions further north in the image on the right. In this enhanced color view, the blue “tiger stripes” stand out. The “tiger stripes” are fissures that spray icy particles, water vapor and organic compounds.

9.8.5 Triton

Neptune’s large moon, Triton, is about 80% the diameter of the Moon and has a density of 2.1 times the density of water. It is probably made of about 75% rock surrounded by a thick layer of frozen water. Triton is even colder than Pluto, about 35 to 40 K. Because of the extremely cold temperatures, nitrogen can be frozen on its surface. On other moons and planets, nitrogen is a gas. Some of the nitrogen on Triton is gaseous and makes up its thin atmosphere. Triton has a young surface with smooth frozen lakes and cantaloupe-textured terrain of unknown cause.

Triton has many black streaks on its surface that may be from volcanic venting of nitrogen heated to a gaseous state despite the very low temperatures by high internal pressures. The nitrogen fountains are about 8 kilometers high and then move off parallel to the surface by winds in the upper part of its thin atmosphere. Another unusual thing about Triton is its highly inclined orbit (with respect to Neptune’s equator). Its circular orbit is retrograde (backward) which means the orbit is decaying—Triton is spiraling into Neptune. Triton’s strange orbit and the very elliptical orbit of Neptune’s other major moon, Nereid, leads to the proposal that Triton was captured by Neptune when Triton passed too close to it. If it was not captured, Triton was certainly affected by something passing close to the Neptune system.

9.8.6 Rings

All of the Jovian planets have a system of rings. Jupiter has four faint rings: a flattened main ring, a puffier inner ring, and two wispy outer rings that are inside the orbit of Io. The rings are made of very small, dark particles the size of smoke particles. They are produced by dust kicked up from the tiny innermost moons of Jupiter by impacts on the moons.

Upper left-center is the Galileo spacecraft’s view of Jupiter’s faint rings when looking back toward the Sun—the best position for viewing very faint rings made of tiny particles. The lower right graphic is a description of the rings and where they are with respect to some small moons very close to Jupiter. Jupiter’s ring system is composed of three parts: an outermost gossamer ring, a flat main ring, and an innermost donut-shaped halo.

9.8.6.1 Saturn’s Rings

The planet with the spectacular ring system is Saturn. Icy particles spread out into large, flat rings make up Saturn’s ring system that can be seen with even low-power telescopes on the Earth’s surface. The rings of the other Jovian planets are dark and faint, so they were discovered only relatively recently with either powerful telescopes or by spacecraft flybys.

Saturn’s rings were discovered by Christian Huygens in 1659. Galileo’s telescope was too small to make them look like more than just a couple of bumps on either side of the planet. In 1675 Giovanni Cassini discovered a gap between the two large (A & B) rings, now called the Cassini division in his honor. With improved telescopes, astronomers were able to see that one of the large rings was in fact, two rings (B & C) and there is a gap in the A ring (the Encke division). There is also a hint of another ring closer to the planet than the C ring (the D ring). When the Pioneer and Voyager spacecraft flew by, astronomers found more rings and complex structure in the rings.

The rings that are visible in even low-power telescopes on the Earth (A, B, and C) extend from about 74,000 kilometers to about 137,000 kilometers from Saturn’s center (or 1.23 to 2.28 Saturn radii). The rings are very thin, less than a hundred meters thick. A scale model of the rings with the width equal to a single piece of regular paper would be about 100 meters across! Collisions between the ring particles keeps the ring system very flat and all of the particle orbits circular. In 1859 James C. Maxwell (of electromagnetism fame), showed that the rings could not be solid, but, rather a swarm of particles. A solid ring the width of Saturn’s ring system would become unstable and break up. James Keeler proved Maxwell correct in 1895 when he measured the Doppler shifts of different parts of the rings and found that the outer parts of the ring system orbited at a slower speed than the inner parts. The rings obeyed Kepler’s third law and, therefore, must be made of millions of tiny bodies each orbiting Saturn as a tiny mini-moon. Spectroscopy of the rings shows that the particles are made of frozen water.

Computer-generated picture of the particles in a 3-meter square section of Saturn’s A ring.

More recently, astronomers bouncing radar off the rings and analyzing the reflected signal found that ring particles must be from a few centimeters to a few meters across. When the Voyager spacecraft went behind the rings with respect to the Earth, astronomers could measure the particles sizes from how Voyager’s radio signal scattered off the particles and from how sunlight scattered through the rings. The ring particles range in size from the size of a small grain of sand to the size of a large house, but on average, they are about the size of your clenched fist. The effect of particle size on how the particles scatter sunlight is nicely illustrated in the iconic image below as well as in a movie made by Cassini crossing the ring plane from above to below. The Sun is behind Saturn and Cassini is in Saturn’s shadow. Very small particles will scatter sunlight forward while larger particles will scatter the sunlight backward. Recall how the very fine dust on the inside of your car’s windshield will make it nearly impossible to see through it when you are driving toward the Sun. People on the outside of your car are able to easily see you inside through your windshield because there is not any backscattering of the sunlight. They would have a much harder time seeing you though, if the outside of your windshield was coated with grittier dust and dirt. The Voyager and Pioneer spacecraft confirmed the presence of the inner D ring and discovered two other rings beyond the outer A ring: a narrow F ring just outside the A ring and a broad, but faint E ring. Cassini added another two rings with this image: a ring that shares the same orbit as Janus and Epimetheus and another that shares the same orbit as Pallene. Micro-meteorite impacts on these moons chip off material to feed their namesake rings. The E-ring is supplied by the geysers of Enceladus.

The broad rings we see from the Earth are actually systems of thousands of tiny ringlets each just a few kilometers wide, so the rings look like grooves in a phonograph record (youngsters only familiar with CDs will have to ask their parents or grandparents about them). Voyager also found some unusual things in Saturn’s ring system: rings that change shape, eccentric ring shapes (some even twist around each other to make a braid), and dark features that look like spokes extending radially outward across the rings.

The grooved pattern of Saturn’s rings are probably the result of spiral density waves forming from the mutual gravitational attraction of the ring particles. Narrow gaps in the rings are swept clean of particles by small moonlets embedded in the rings. The tiny moons can also act as shepherd satellites. Two shepherd satellites with one orbiting slightly outside the other satellite’s orbit can constrain or shepherd the ring particles to stay between the moonlet orbits. The narrow F ring is the result of two shepherd satellites, Pandora and Prometheus, between 80 and 100 kilometers across with orbits about 1000 kilometers on either side of the F ring. Prometheus and Pandora also responsible for the braids and kinks in the F ring. The dark drapes in the image below were created by Prometheus as its gravity tugged on F ring material at its closest approach to the F ring. See the movie of this in process by selecting the picture.

Saturn’s A and B rings are separated by the Cassini Division. The Encke Gap is in the outer part of the A-ring. Darker streaks (spokes) extending across the B-ring are also visible.

Bigger gaps in the rings (such as the Cassini division) are the result of gravitational resonances with the moons of Saturn. A resonance happens when one object has an orbital period that is a small-integer fraction of another body’s orbital period, e.g., 1/2, 2/3, etc. The object gets a periodic gravitational tug at the same point in its orbit. Just as you can get a swing to increase the size of its oscillation by pushing it at the same point in its swing arc, a resonance can “pump up” the orbital motion of an object. Particles at the inner edge of the Cassini division are in a one-two resonance with the moon called Mimas—they orbit twice for every one orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap. Other resonances with Mimas are responsible for other features in Saturn’s rings: the boundary between the C and B ring is at the 1:3 resonance and the outer edge of the A ring is at the 2:3 resonance. It is amazing what a simple inverse square law force can do!

The dark spokes in Saturn’s B ring were a surprise. The different orbital speeds of the ring particles should quickly shear apart any radial structure in the rings, but the spokes clearly survived the shearing! The spokes are probably caused by very tiny dust particles hovering just above the rings by their interaction with Saturn’s magnetic field or by electrostatic forces created from ring particle collisions. The spokes look dark because they are scattering light in the forward direction (away from the spacecraft) and are most easily seen near Saturn’s equinox times that occur roughly every 15 years.

Where did the rings of Saturn come from? Studies of the various forces on the ring particles show that the rings are transient—they did not form with Saturn as part of the formation of the main planet, nor will they always be there. The rings of Saturn are within the distance at which a large moon would experience extreme enough tidal stretching to be torn apart. This distance is called the Roche limit, after M.E. Roche who developed the theory of tidal break up in the 1849. The exact distance of the Roche limit depends on the densities of the planet and close-approaching moon and how strongly the material of the moon is held together. The classical Roche limit considers a moon held together only by its internal gravity. Such a moon would break up at a distance of about 2.44 planetary radii from the center of the planet.

Saturn’s rings lie within Saturn’s Roche limit so it is likely that they were formed by particles too close to Saturn to ever form a large moon. The fact that all of the Jovian planets have rings argues against the breakup of a large moon spiraling in toward Saturn from a rare encounter with an object passing through the moon system. (Maybe it could happen with one planet, but with all four Jovians?) Another possibility is that large collisions on the large moons outside the Roche limit spewed material into the region inside the Roche limit. Small moonlets are able to exist in the rings because the tidal stretching across their small diameters are too small. Tidal forces will prevent them from getting large. Collisions among themselves and micro-meteorite impacts can replenish a ring. Some the ring material may be able to clump together to create the moonlets and that may be why some of the moonlets have significant equatorial ridges to make them look like “flying saucers”. (Are these pictures making the rounds on the internet as “proof” of aliens?)

Saturn’s E ring lies outside Saturn’s Roche limit and is most concentrated at the orbit of the icy moon, Enceladus. Eruptions of water vapor from Enceladus are the source of the E ring material. The image below shows the geysers erupting out of Enceladus’ south polar region (extra bright material below the dark circle of Enceladus) and the shadow of Enceladus casting onto the E-ring material in which Enceladus orbits.

A more informative way to compare the planet ring systems: scale them relative to the diameter of each planet.

9.8.6.2 The Rings of Uranus and Neptune

Uranus’ rings were discovered in 1977 from measuring the intensity of light from a star as Uranus passed in front of it. Astronomers were originally intending to learn about Uranus’ atmosphere from how the light passed through the top cloud layers and to measure both the diameter of Uranus and the star accurately from timings of when the star was totally blocked. They noticed that the star blinked off and on before Uranus itself passed in front of the star. The star also blinked off and on after Uranus itself had moved out of the way. The symmetry of the winking out of the star as the rings passed in front of the star pointed to their existence. Later infrared observations gave astronomers more information about them. Neptune’s rings were discovered in the same way a few years after the Uranus’ rings discovery. Voyager 2 gave a much better view of the rings when it flew by Uranus in 1986 and Neptune in 1989.

Hubble Space Telescope view of Uranus, its four major rings and ten of its satellites with its near-infrared camera. A visible light image of the rings would not be so impressive. Colors of Uranus indicate altitude: green and blue are the deeper layers, yellow and grey are higher haze and clouds, and orange and red are very high cirrus clouds.

Voyager 2’s view of Uranus’ rings when seen looking back toward the Sun. This image has been colorized: real color is gray and they are as dark as charcoal. See the image link for the how this image was processed.

The rings of Uranus and Neptune are much darker than Saturn’s rings, reflecting only a few percent of what little sunlight reaches them (they are darker than pieces of black, burned wood) while Saturn’s rings reflect over 70% of the Sun’s light. The rings are also much narrower than Saturn’s rings. Uranus’ outermost and most massive ring, called the Epsilon Ring, is only about 100 kilometers wide and probably less than 100 meters thick. The other ten dark and narrow rings have a combined mass less than the Epsilon Ring. The six rings of Neptune are less significant than Uranus’ and the ring particles are not uniformly distributed in the rings. Like Saturn’s F ring, the rings of Uranus and Neptune are kept narrow by shepherd satellites. The narrowness and even clumpiness of the rings means that the rings can last for only a short time—a million years or so, unless the rings are replenished by material ejected off the moons in large collisions.

Neptune’s rings as seen by Voyager 2 when looking back toward the Sun. Bright Neptune has been blocked out with the center bar. Two different exposures are combined for this image, each 591 seconds long and separated by 1 hour 27 minutes.

Sections Review

Vocabulary

• giant impact theory
• mare
• resonance
• Roche limit

Review Questions 7

1. What do the mare look like on the Moon and why are they so smooth?
2. Why does the Moon not have erosion?
3. What two reasons explain why the Moon is geologically dead?
4. In what ways are the Moon and Mercury like each other? In what ways are they different from each other?
5. In what ways are the Moon and the Earth like each other? In what ways are they different from each other?
6. In what ways is the giant impact theory better at explaining the formation of the Moon than other Moon formation theories?
7. Why is Io such a geologically-active moon? Describe how its interior is kept molten.
8. What is the interior of Europa like?
9. What is so unusual about Europa’s surface?
10. Why is Europa another good place to look for life beyond the Earth?
11. What are four lines of evidence for a liquid water ocean below Europa’s surface? Which line is shared with Ganymede and Callisto?
12. What causes the parallel ridges on Ganymede?
13. What about the compositions of the Jovian planets’ moons enable them to be geologically active despite their small size?
14. What is Callisto like and how do you know the age of its surface?
15. Compare/contrast Titan with our Moon (size, mass, surface conditions, etc.)
16. What is so special about Titan?
17. What is Titan’s atmosphere made of and why is it so thick?
18. How is Titan probably like the early Earth?
19. In what ways is Titan like Earth today?
20. What is a recent surprising discovery about Enceladus? Why is it another possible place to look for life beyond the Earth?
21. How cold is Triton and what is it made of?
22. Is Triton’s surface young or old and what causes some of Triton’s strange surface features?
23. What is unusual about Triton’s orbit?
24. What are Saturn’s rings made of and how do we know?
25. How do we know the sizes of the particles in Saturn’s rings?
26. How are grooves and gaps made in the rings of Saturn?
27. What unusual things were found in Saturn’s rings and what are the likely causes?
28. What formed the rings of Saturn and when were they made?
29. Compare/contrast the rings of Uranus and Neptune with Saturn’s rings.

Is this page a copy of Strobel’s Astronomy Notes?