The mineral apatite comes from the Greek word ‘apat,’ which means deceitful. Greenish in nature with several variations, apatite is often mistaken as a precious gem. The mineral also apparently tricked scientists into thinking there was more water on the moon than their actually was. However, once a team of once-rival scientists, including University of New Mexico Senior Research Scientist Francis McCubbin, decided to join forces, the trickery was over.
For decades, scientists believed there was no water on the moon. However, the discovery of a hydrogen-rich mineral called apatite within lunar rocks in 2010, which involved several of the collaborators in this research, seemed to hint at a watery past where water abundances were nearly as high on earth. 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, the team, led by Jeremy Boyce of the UCLA Department of Earth, Planetary and Space Sciences, discovered that apatite was probably deceiving scientists all along.
"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."
Along with rivals McCubbin and James Greenwood, and co-authors Steve Tomlinson of UCLA and Allan Treiman from the NASA-funded Lunar and Planetary Institute, which supported the research along with a NASA Cosmochemistry grant, they simulated the formation of apatite minerals containing different amounts of volatile elements - hydrogen, chlorine and fluorine - by creating a computer model to accurately predict how the mineral would have crystallized from cooling bodies of lunar magma early in the moon’s history.
They demonstrated that it was possible to start with any water composition in the magma and by varying only the degree of crystallization and the chlorine content, reproduce all the features seen in a diverse range of apatite from the moon. 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 scientists originally expected.
"We used to think it was a simple proportionality - that the more hydrogen was in the apatite, the more hydrogen in the magma,” Boyce said. "Then we figured out… that it's a competition between hydrogen and mostly fluorine. Fluorine is the element that apatite most wants."
"There is some hydrogen that's coming in from the solar wind and getting stuck on the surface,” McCubbin said. “But there is some amount - definitely seems to be less than on earth - that the moon started with."
This discovery has overturned the long-held assumption that the hydrogen in apatite is a good indicator of overall lunar water content. The scientists believe the high water content within lunar apatite results from a quirk in the crystallization 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 crystallizing mineral only if apatite's preferred building blocks, fluorine and chlorine, have been mostly exhausted.
“The experiments we were conducting indicated a clear preference for F and Cl over OH” said McCubbin. “However, during a process called fractional crystallization, the earliest formed apatites vacuum the fluorine and chlorine out of the magma leading to the crystallization of water-rich apatites that tricked us into thinking the moon was as wet as the earth. Apatite is often mistaken as a precious gem and it’s known for being deceitful, but this is the last time we're going to let it trick us.”
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.
"Apatite that forms later doesn't see any fluorine or chlorine and becomes hydrogen-rich because it has no choice," Boyce said.
Understanding the story of lunar apatite has implications beyond determining how much water is locked inside lunar rocks and soil. According to the widely-believed theory of how the moon originally formed, hydrogen and other volatile elements should not be present at all in lunar rocks. This study also shows that scientists still have much to learn about the composition and environment of the early moon.
This recent discovery might never had happened if the scientists didn’t collaborate with each other. For years, the trio of Boyce, Greenwood and McCubbin were fierce competitors. They had been publishing papers on lunar apatite and each raced individually to get their research published first. However, instead of continuing to be research rivals, one night after a scientific conference and a beer, the team decided they could produce better research, and maybe solve a decades-old problem if they collaborated.
“We were fierce competitors, we got into heated debates” said McCubbin. “We came to the conclusion that we would be stronger working as a team, than as separate researchers.”
And so the collaboration began, which led to stronger research and deciphering the deceitful mineral apatite. The results, so far, speak volumes for the team.
To read the abstract from the research published recently in Science, visit: The Lunar Apatite Paradox.