Watery Magma From The Moon

A new study of preserved bubbles of magma trapped in ancient moon rocks shows the magma contains 100 times more water than that found in previous moon rock analyses and carries a profile of volatiles closely resembling those found in Earth’s upper mantle.

This finding may require a retooling of theories of the moon’s formation, say geochemist Erik H. Hauri, at the Carnegie Institution of Washington; Alberto E. Saal, geological sciences professor at Brown University; James Van Orman, geological sciences professor at Case Western Reserve University; and their colleagues, who performed the study (Science, DOI: 10.1126/science.1204626).

The reigning theory of moon formation, called the giant-impact theory, holds that a Mars-sized chunk of rock slammed into the nascent Earth billions of years ago, expelling debris that quickly lost its volatile compounds and formed a dry moon. Using the more sophisticated analytical technology developed in the past decade, however, several recent studies of moon rocks have found more water than expected.

In the new study, the researchers focused on a sample, brought back in 1972 by astronauts on Apollo 17, known as “orange soil” for its stark orange contrast with typical grayish moon rocks. The orange soil was formed at the foot of a lava fountain around 3.6 billion years ago.

The group used ion microprobe analysis to look at the magma melt bubble trapped inside the orange-soil crystals. In addition to the startling high levels of water, the samples also contain high levels of fluorine, sulfur, and chlorine. The group says that to explain the findings scientists will need to come up with mechanisms by which volatiles could have been retained during the moon’s formation.

“This work is definitely an important element for all the theories of the origin of the moon,” says Francis Albarède, geochemistry professor at École Normale Supérieure in Lyon, France. “Of course, this great finding should be treated with some caution,” he notes, pointing out that the orange soil is unique among hundreds of lunar samples brought back by the Apollo missions and may represent only local conditions.

Other possible mechanisms that don’t necessarily interfere with the giant-impact theory could be invoked to explain the samples’ water content, Albarède adds, including impacts from volatile-rich asteroids fertilizing the local mantle with water.

Chemical & Engineering News

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