The damp moon: team finds water on lunar surface: interior volcanic rocks also hold more [H.sub.2]O than thought.
Observations from three spacecraft suggest that water molecules are widely distributed over a thin layer of the lunar surface rather than locked up in icy enclaves predicted to lie at the moon's poles. The results, detailed in a trio of papers posted online September 24 in Science, suggest that liquid water may be more available to future moon explorers than had been thought. Concentrations in sunlit soil might reach about 1,000 parts per million, the equivalent of roughly a quart of water per ton of material. That water doesn't remain on the moon, but comes and goes each lunar day.
In contrast, water molecules bound to phosphate minerals within volcanic rocks--material that formed well beneath the lunar surface--date back several billion years, says Francis McCubbin of the Carnegie Institution for Science in Washington, D.C. A fourth, unpublished study led by McCubbin finds a surprisingly high abundance of this interior water, which may shed new light on how the moon formed.
The researchers who made the surface observations caution that their observations cannot clearly distinguish between water and the hydroxyl ion. which can serve as a marker for water.
Nonetheless, Roger N. Clark of the U.S. Geological Survey in Denver asserts that "this is the first detection of water on the moon and we see it all over, not just in the polar regions." Clark, a coauthor of two of the papers in Science, led a team that found evidence of water in spectra taken by the Cassini spacecraft in 1999. Clark says he waited years to publish because "the detection was so fantastic, I felt we needed confirmation."
Confirmation has now come in the form of spectra taken by instruments aboard NASA's Deep Impact spacecraft and Chandrayaan-1, India's first lunar mission. On September 17, other researchers reported that the Lunar Reconnaissance Orbiter spacecraft had found hydrogen on the moon's surface, another possible marker of water (SN Online: 9/18/09).
The three papers "present a strong case for surficial water on the moon, and this could certainly be the result of delivery by icy impactors or solar wind interactions long after the moon formed," comments Robin Canup of the Southwest Research Institute in Boulder, Colo.
In McCubbin's study of the lunar interior, he and his colleagues calculate that phosphate minerals recovered from the moon contain a concentration of water as high as several thousand parts per million. This result, combined with lower abundances reported for other volcanic material, points to an average overall abundance of water in the lunar mantle significantly higher than the previous estimate of 1 part per billion.
It has been a long-standing assumption, notes Canup, that if the moon formed when a giant impactor smacked into the young Earth, any water would have been vaporized by the high temperatures generated and that vapor would have escaped into space. However, that assumption "has yet to be evaluated with direct models," she adds.
McCubbin agrees that there may have been some way for water to be retained in this impact model. Alternative explanations would have to account for the water now known to reside inside the moon.
Says Canup: "Our picture of abone-dry moon is cleartyin need of updating."
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|Title Annotation:||Atom & Cosmos|
|Date:||Oct 24, 2009|
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