Moon May Not Have Much Water On It After All
For years it was believed that the Moon was parched and drier than the driest desert on Earth. Then with the discovery of the mineral apatite, it was believed that water did exist on the surface of the Moon. But now a team of researchers believe that scientists studying the mineral may have been misled about the amount of water present on the moon. According to a press release Wednesday, the research was published on March 20 in the online journal Science.
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Analysis of the moon rock apatite revealed water trapped in the mineral as hydroxyl leading to the hypothesis that if the mineral-locked water was hypothetically converted to liquid, it would cover the moon's surface in roughly one meter of water.
But re-analysis of the mineral by Jeremy Boyce of the UCLA Department of Earth, Planetary, and Space Sciences and his team has contradicted this assumption that the hydroxyl in apatite is a good indicator of overall lunar water content. Their theory is based on computer models that were used to simulate the crystallization of apatite from cooling bodies of lunar magma early in the formation of the Moon. They found that the 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, 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."
When apatite was first discovered within lunar rocks in 2010, scientists believed that the mineral was indicative of the water content of a large body of magma, or even the entire moon. But Boyce's study reveals that apatite was hugely misrepresentative.According to Boyce the high water content within lunar apatite may not be due to water on the surface of the moon but due to an aberration in its crystallization process.When molten rock cools, apatite incorporates the hydrogen molecules which are present into its crystal structure. But hydrogen is preferred only when concentrations of the other important minerals, fluorine and chlorine have been depleted.
"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, in the presence of fluorine and chlorine, a cooling body of magma forms hydrogen-poor apatite and once the other minerals are exhausted it forms hydrogen-rich apatite.
The study of apatite goes much beyond determining the lunar water content. If the popular theory of how the moon was formed is to be considered then hydrogen and other volatile elements should be absent from the lunar surface. The theory is that the moon formed when a giant meteor hit the Earth and a huge chunk of its surface got separated. If this "giant impact" model is correct, the moon would have been completely molten, and lighter elements such as hydrogen should have bubbled to the surface and escaped into space. Since hydrogen is needed to make water, a moon formed by a giant impact would be waterless.
And this may even be true since a host of lunar materials are dry and devoid of lighter elements. Yet the discovery of apatite in so many samples proves that despite the moon's early molten state some water and other volatiles may have remained but not as much as apatite led us to believe.
"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."
Researchers are nowhere close to unravelling the secrets of the moon and this research is only a stepping stone in finding out about its past. "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."
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