The amount of water present in the Moon may have been overestimated by scientists studying the lunar rock samples, a new study has claimed.
Researchers led by Jeremy Boyce from the University of California - Los Angeles created a computer model to accurately predict how mineral apatite would have crystallised from cooling bodies of lunar magma early in the Moon's history.
Their simulations showed 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.
This discovery has overturned the long-held assumption that the hydrogen in apatite is a good indicator of overall lunar water content.
"The mineral apatite is the most widely used method for estimating the amount of water in lunar rocks, but it cannot be trusted," said Boyce.
"Our new results show that there is not as much water in lunar magma as apatite would have us believe," said Boyce.
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.
Scientists originally assumed that information obtained from a small sample of apatite could predict the original water content of a large body of magma, or even the entire Moon, but Boyce's study indicates that apatite may, in fact, be deceptive.
Boyce believes 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.
However, hydrogen will 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 doesn't see any fluorine or chlorine and becomes hydrogen-rich because it has no choice," said Boyce.
Therefore, when fluorine and chlorine become depleted, a cooling body of magma will shift from forming hydrogen-poor apatite to forming hydrogen-rich apatite, with the latter not accurately reflecting the original water content in the magma.
Understanding the story of lunar apatite has implications beyond determining how much water is locked inside lunar rocks and soil, researchers said.
The study was published in the journal Science.
