Lunar liquid

Early on, the moon’s interior may have been as wet as Earth’s upper mantle

11:55am, July 9, 2008
Magazine issue: Vol. 174 #3, August 2, 2008
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A new analysis of moon rocks has revealed that the moon isn’t as bone dry as researchers had thought, whetting the appetite of scientists who seek a deeper understanding of how Earth’s natural satellite arose and evolved.

Because the moon is believed to have coalesced from the debris created when a Mars-sized body struck Earth some 4.5 billion years ago, the finding also suggests that Earth acquired a substantial supply of water earlier in its history than some scientists had suspected.

The moon rocks were brought back to Earth by the Apollo astronauts, and for three decades measurements of tiny, volcanically formed glass beads in the rocks showed no signs of water. It took three years to convince NASA to re-examine the rocks with a more sensitive instrument using a narrow beam of cesium ions, says Alberto Saal of BrownUniversity in Providence, R.I.

The new measurements reveal that the concentration of water in the rocks is less than 50 parts per million — below the limit that could have been detected with less sensitive ion probes, notes coinvestigator Erik Hauri of the Carnegie Institution of Washington (D.C.).

A critical finding, he notes, is that the concentration of water decreases dramatically from the center to the rim of the beads. That decline suggests that 95 percent of the water once held by the moon was lost when volcanic eruptions belched water vapor and other volatile gases. The lunar interior may originally have had an abundance of water approaching 750 ppm — similar to the amount present in Earth’s upper mantle, Saal’s team reports in the July 10 Nature.

“This work challenges the long-standing assumption that the moon lacks indigenous water, and this result alone makes it very important to understanding the origin and early history of the moon,” comments theorist Robin Canup of the Southwest Research Institute in Boulder, Colo.

Saal cautions that it’s unclear if all parts of the moon had similar concentrations of water, or if the beads only indicate the amount of water in a small section represented by the rocks.

Answering that puzzle “will be key to unraveling what this result implies,” Canup says. For instance, if the abundances reflect an average lunar composition, then the total mass of water originally contained by the moon would be larger than could have been delivered by water-rich meteoroids that pelted the lunar surface during the era known as the late heavy bombardment some 3.8 billion years ago.

But if the average amount of lunar water is much lower, the finding may cast a new light on the standard model of moon formation, says Saal. According to that model, the heat generated by the cataclysmic collision between the fledgling Earth and the Mars-sized body that generated the moon would have vaporized any water.

One possibility is that Earth had acquired much of its water before the giant impact, and that the Earth-moon system somehow managed to retain significant amounts after the collision, Saal and his colleagues suggest. In fact, few models directly address exactly how much water would be lost or retained during the moon’s formation, Canup says. Alternatively, Saal’s team proposes, water-rich material may have been accumulated by both the Earth and the moon during a narrow window of time, less than 100 million years, after the moon-forming impact.

On Earth, “water very likely existed in abundance” by the time the planet finished forming about 4.45 billion years ago, notes David Stevenson of the California Institute of Technology in Pasadena. The possibility of life on Earth before the era of late heavy bombardment is “not an unreasonable idea and not new,” he adds. The lunar water does not offer a haven for life on the moon, notes Stevenson, because that water is trapped inside rocks and any that escaped could not accumulate in substantial amounts on the moon’s surface because of the low gravity and lack of atmosphere.

The new study suggests that some of the water-ice that may reside within permanently shadowed craters at the lunar poles could have come from within the moon rather than by delivery from comets and meteoroids. Two NASA craft, the Lunar Reconnaissance Orbiter, set for launch later this year, and the Lunar Crater Observatory and Sensing Satellite, scheduled for 2009, will look for that frozen water.

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