April 2, 2014
Apatite May Not Actually Be A Good Indicator Of Water On The Moon
April Flowers for redOrbit.com - Your Universe Online
Scientists studying the mineral apatite may have overestimated the amount of water present in the moon, according to a new study led by the University of California Los Angeles (UCLA) Department of Earth, Planetary and Space Sciences.In order to accurately predict how apatite would have crystallized from cooling bodies of lunar magma early in the moon's history, Jeremy Boyce led a team of researchers to develop a new computer model. The model's simulations demonstrated that the long held belief that the hydrogen in apatite is a good indicator of overall lunar water content by showing that the unusually hydrogen-rich apatite crystals found in many lunar rocks might not have formed in a water-rich environment.
"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, who is an adjunct assistant professor in the UCLA College of Letters and Science. "Our new results show that there is not as much water in lunar magma as apatite would have us believe."
Scientists have believed the moon was almost completely devoid of water for decades. In 2010, however, hydrogen-rich apatite was discovered in lunar rocks, hinting at a more watery past for the moon. Prior studies made the assumption that data obtained from a small sample of apatite could predict the original water content for a large body of magma, if not the entire moon. The new findings suggest that the apatite may be deceiving, however.
A quirk in the crystallization process is to blame for the hydrogen-rich apatite in the lunar rocks, according to Boyce, not a water-rich environment. Apatite only forms with hydrogen when its preferred building blocks, fluorine and chlorine, are absent.
"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."
Cooling magma will shift from forming hydrogen-poor apatite (when there is plenty of fluorine and chlorine) to hydrogen-rich apatite (when the fluorine and chlorine are depleted). Despite past assumptions, Boyce says that hydrogen-rich apatite is not a reliable predictor of the original water content of the magma.
Solving the mystery of the hydrogen-rich apatite has implications beyond determining the amount of water locked inside lunar rocks and soil. Hydrogen, and other volatile elements, should not be present in lunar rocks at all, if the predominant theory of the moon's formation is correct.
The theory holds that the moon formed when a giant impact tore free a large chunk of the Earth, approximately 4 billion years ago. Under these conditions, the moon would have been completely molten, and lighter elements such as hydrogen should have bubbled to the surface and escaped into space. Thus, a moon formed by a giant impact should be dry, since hydrogen is a key component in water.
The fact is, most lunar rock samples are very dry, and are missing those lighter elements. The hydrogen-rich apatite crystals found in a wide range of lunar samples present a paradox for scientists. The crystals suggest that some water and other volatiles may have remained after the fiery beginning of the moon, just not as much as the mineral initially implied.
"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."
The most important finding of this study is 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."
The findings were recently published online in Science and will be published in a future print edition.
Image 2 (below): The image is a map of calcium within a polished thin slice of a lunar rock. The high calcium content in apatite is indicated by the bright pinks and reds, while surrounding minerals with lower calcium content are shown in blues and black. Core to rim zoning of water in crystals such as this one demonstrates the presence of the apatite fractionation, an effect responsible for the high water content of lunar apatites on an otherwise dry moon. Credit: UCLA