Water on moon

Print edition : September 20, 2013

Scientists have learned that the lunar impact crater Bullialdus has significantly more hydroxyl, a molecule of one oxygen atom and one hydrogen atom, than its surroundings. Here, the central peak of Bullialdus rising above the crater floor with the crater wall in the background. Photo: NASA

SCIENTISTS have detected magmatic water—water that originates from deep within the moon’s interior—rather than the water- and hydroxyl-bearing igneous lunar material detected by lunar missions thus far from the surface. These findings, published in the August 25 issue of Nature Geoscience, represent the first such remote detection of water that is indigenous to the moon and were arrived at using data from the NASA’s Moon Mineralogy Mapper (M3), which was flown aboard Chandrayaan-1. Hitherto researchers believed that the rocks from the moon were “bone dry” and that any water detected in the lunar samples had to be contamination from the earth and other extra-lunar sources. “About five years ago, new laboratory techniques used to investigate lunar samples revealed that the interior of the moon was not as dry as we previously thought. Around the same time, data from orbital spacecraft detected water on the lunar surface, which is thought to be a thin layer formed from solar wind hitting the lunar surface,” said Rachel Klima, a planetary geologist at Johns Hopkins University, a member of the NASA Lunar Science Institute’s Scientific and Exploration Potential of the Lunar Poles team and the lead author of the paper.

One may recall that the Moon Impact Probe (MIP)—that is, the CHACE (Chandra Altitudinal Composition Explorer) it carried—aboard Chandrayaan-1 detected signs of hydroxyl and water molecules first, about 10 months before M3 did. “This surficial water unfortunately did not give us any information about the magmatic water that exists deeper within the lunar crust and mantle, but we were able to identify the rock types in and around the Bullialdus crater,” said co-author Justin Hagerty, of the U.S. Geological Survey. “Such studies can help us understand how the surficial water originated and where it might exist in the lunar mantle.” In 2009, the M3 fully imaged the lunar impact crater Bullialdus. “It’s within 25 degrees latitude of the equator and so not in a favourable location for the solar wind to produce significant surface water,” Rachel Klima explained. “The rocks in the central peak of the crater are of a type called norite that usually crystallises when magma ascends but gets trapped underground instead of erupting on the surface as lava. Bullialdus crater is not the only location where this rock type is found, but the exposure of these rocks combined with a generally low regional water abundance enabled us to quantify the amount of internal water in these rocks.”

After examining the M3 data, Rachel Klima and her colleagues found that the crater had significantly more hydroxyl presence than its surroundings. “The hydroxyl absorption features were consistent with hydroxyl bound to magmatic minerals that were excavated from depth by the impact that formed Bullialdus crater,” she writes. Furthermore, estimates of thorium concentration in the material of the central peak indicate an enhancement in incompatible elements, in contrast to the compositions of water-bearing lunar samples. “Now we need to look elsewhere on the moon and try to test our findings about the relationship between the incompatible trace elements (like thorium and uranium) and the hydroxyl signature,” Rachel Klima said.

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