How moist is the moon? Not very, say scientists

A new study suggests that previous estimates overestimated the amount of water on the moon.

There is probably less water on the moon than previously thought. 

According to a new study, the findings of which are published in a paper titled "The Lunar Apatite Paradox" in the journal Science, the most widely used method for estimating the amount of water in lunar rocks could be deceptive, say researchers.

A method that examines the mineral apatite for estimating the amount of water in lunar rocks cannot be trusted, said lead author and UCLA geochemist Jeremy Boyce. "Our new results show that there is not as much water in lunar magma as apatite would have us believe," he said in a press release.

Apatite, a common mineral found in terrestrial rocks, as well as in our bones and teeth, can also be found the moon. It crystallizes from lunar magma, says Allah H. Treiman, a scientist at the NASA-funded Lunar and Planetary Institute in Houston and an author on the paper.

After examining samples of rocks that were retrieved by various Apollo sample return missions, many scientists had first predicted that the moon's environment was very dry. But in 2010, after analyzing hydrogen-rich apatite within lunar rocks, some scientists suggested that it was possible for the moon to have a water-rich environment.

But this method could be misleading, because the hydrogen content within apatite crystals comes from a quirky crystallization process, Dr. Boyce said, who along with his team created computer models to study how apatite crystallized. 

Apatite is a funny mineral, says Boyce, in that if it has to choose between incorporating  fluorine, chlorine and hydrogen, it will choose first fluorine, then chlorine, and the last would be hydrogen. 

Only if fluorine and chlorine are exhausted will the crystal start drawing in large amounts of hydrogen from the magma.

"Early-forming apatite is so fluorine-rich that it vacuums all the fluorine out of the magma, followed by chlorine," Boyce said. "Apatite that forms later doesn't see any fluorine or chlorine and becomes hydrogen-rich because it has no choice."

 So the absence of fluorine and chlorine will result in the formation of hydrogen-rich apatite, but that does not necessarily imply a damp moon.

The current finding lends support to the so-called giant impact hypothesis of the moon's formation. According to this idea, a body roughly the size of Mars smashed into the Earth about 4.5 billion years ago, tearing free a large piece of our planet, which ultimately became the moon.

After the impact, the moon would be largely molten rock, allowing lighter gases such as hydrogen to escape into space. This would explain why there is less hydrogen on the moon than on Earth, and why the amount of water would be less, says Boyce.

"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."