Scientists are puzzled over the water content of the Moon:

The amount of water present in the moon may have been overestimated by scientists studying the mineral apatite, says a team of researchers led by Jeremy Boyce of the UCLA Department of Earth, Planetary and Space Sciences.

Boyce and his colleagues created a computer model to accurately predict how apatite would have crystallized from cooling bodies of lunar magma early in the moon’s history. Their simulations revealed that the unusually hydrogen-rich apatite crystals observed in many lunar rock samples may not have formed within a water-rich environment, as was originally expected.

The prevailing model for how the Moon formed involves a Mars-sized planet crashing into the Earth, ejecting bits of the Earth into space, and some of these bits coalescing into what is now the Moon. If that’s the case, then the Moon should be similar in composition to the Earth — except that lighter elements, like hydrogen (which is a component of water), are believed to have escaped the Moon’s relatively weak gravity and drifted off into space, leaving the Moon rather dry.

In 2010, scientists took samples from the Moon and found apatite crystals, which contained a lot of hydrogen, suggesting that the Moon wasn’t as dry as previously thought. Now, however, with scientists concerned that the amount of hydrogen in these crystals overestimates the amount of water on the Moon, the “giant impact” theory of how the Moon formed is called into question:

“We had 40 years of believing in a dry moon, and now we have some evidence that the old dry model of the moon wasn’t perfect,” Boyce said. “However, we need to be cautious and look carefully at each piece of evidence before we decide that rocks on the moon are as wet as those on Earth.”

This study shows that scientists still have much to learn about the composition and environment of the early moon.

“We’re knocking out one of the most important pillars of evidence regarding the conditions of the formation and evolution of the moon,” Boyce said. “Next, we plan to determine how badly apatite has distorted our view of the moon and how we can best see past it to get at the moon’s origin.”

This is how science works. New evidence refines — or sometimes completely blows away — a theory/hypothesis. You can never be so wedded to an idea that you overlook important evidence that contradicts your model. It’ll be interesting to see if this latest evidence overturns the giant impact theory, and, if so, what the next favored model will be.