Overturning a long-held assumption, a team of researchers has discovered that the amount of water present on the moon's surface may have been overestimated by scientists studying lunar rock samples.
Led by Jeremy Boyce of the University of California, Los Angeles' department of earth, planetary and space sciences, the study 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 mineral apatite is the most widely-used method for estimating the amount of water in lunar rocks.
"Our new results show that there is not as much water in lunar magma as apatite would have us believe," Boyce announced.
For decades, scientists believed the moon was almost entirely devoid of water.
However, the discovery of hydrogen-rich apatite within lunar rocks in 2010 seemed to hint at a more watery past.
He and his colleagues created a computer model to accurately predict how apatite would have crystallised from cooling bodies of lunar magma early in the moon's history.
According to Boyce, the high water content within lunar apatite results from a quirk in the crystallisation process rather than a water-rich lunar environment.
"When water is present as molten rock cools, apatite can form by incorporating hydrogen atoms into its crystal structure," he explained.
However, hydrogen would be included in the newly-crystallising mineral only if apatite's preferred building blocks, fluorine and chlorine, have been mostly exhausted.
"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 does not see any fluorine or chlorine and becomes hydrogen-rich because it has no choice, he noted.
Therefore, when fluorine and chlorine become depleted, a cooling body of magma would shift from forming hydrogen-poor apatite to forming hydrogen-rich apatite.
The majority of lunar samples are, in fact, very dry and missing lighter elements.
Yet, hydrogen-rich apatite crystals are found in a whole host of lunar samples and have presented a paradox for scientists.
"We had 40 years of believing in a dry moon and now we have some evidence that the old dry model of the moon was not perfect," Boyce added.
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," the researchers said in a paper published in the journal Science.
