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Physical Chemistry

Chlorine Record Suggests The Moon Was Always Dry

A study of chlorine isotopes in moon rocks indicates that Earth's satellite is devoid of water, contradicting other recent studies

by Elizabeth K. Wilson
August 9, 2010 | APPEARED IN VOLUME 88, ISSUE 32

Rock Secrets
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Credit: NASA
Scientists studied chlorine isotopes in "Great Scott," a moon rock collected by astronaut Dave Scott during the Apollo 15 mission.
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Credit: NASA
Scientists studied chlorine isotopes in "Great Scott," a moon rock collected by astronaut Dave Scott during the Apollo 15 mission.

A study of chlorine isotopes in moon rocks brought back from the Apollo missions indicates that Earth’s satellite has always been devoid of water (Science, DOI: 10.1126/science.1192606). This finding contradicts recent studies suggesting that the moon might contain significant amounts of water in its interior (C&EN, June 21, page 27). Zachary D. Sharp of the University of New Mexico and colleagues used mass spectrometry to study the range of ratios of 37Cl to the more common 35Cl in a dozen different moon rock samples. Because chlorine is hydrophilic, it can be used to trace the history of hydrogen, and thus water, in geological samples. The researchers found that the range of isotope ratios is 25 times greater on the moon than on Earth. If the moon had contained significant amounts of water during its early years, volatile HCl would have been produced in volcanic magmas, a process that does not generate a large chlorine isotope fractionation. Instead, Sharp and coworkers reason, the magmas contained almost no hydrogen and instead produced volatile metal chlorides, a process that leads to significant chlorine isotope fractionation, as observed.

Beam Me Up
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Credit: Courtesy of Chip Shearer
An electron microprobe backscatter electron image shows a subsample of the moon rock "Great Scott."
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Credit: Courtesy of Chip Shearer
An electron microprobe backscatter electron image shows a subsample of the moon rock "Great Scott."
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