Erik Hauri (left) and Alberto Saal (right) with the DTM NanoSIMS 50L ion microprobe, the instrument used to make the first measurements of water in lunar melt inclusions. Photo credit: Steve Jacobsen, Northwestern University

The Moon has much more water than previously thought, a team of scientists led by DTM’s Erik Hauri has discovered. Their research paper, published today in Science Express, shows that inclusions of magma trapped within crystals collected during the Apollo 17 mission contain 100 times more water than estimated in prior studies. These results could markedly change the prevailing theory about the Moon’s origin.

The announcement of the discovery was covered by media worldwide, including The New York Times, MSNBC, The Boston Globe, the BBC, and Bloomberg News.  A podcast of Hauri’s interview on National Public Radio’s Science Friday program can be found here.

The research team used a state-of-the-art NanoSIMS 50L ion microprobe to measure seven tiny samples of magma trapped within lunar crystals as so-called “melt inclusions.” These samples came from volcanic glass that contain crystal-hosted melt inclusions. The melt inclusions were prevented from losing their water when explosive volcanic eruptions brought them from depth and deposited them on the Moon’s surface eons ago.

“In contrast to most volcanic deposits, the melt inclusions are encased in crystals that prevent the escape of water and other volatiles during eruption. These samples provide the best window we have to the amount of water in the interior of the Moon,” said James Van Orman of Case Western Reserve University, a member of the team.  The paper’s authors are Hauri; Thomas Weinreich, Alberto Saal, and Malcolm Rutherford from Brown University; and Van Orman.

Backscatter electron image of a lunar melt inclusion surrounded by olivine, from Apollo 17 sample 74220. Skeletal crystals within the inclusion are a fine mixture of olivine and ilmenite. The inclusion is 30 micrometers across. Photo credit: John Armstrong, Geophysical Laboratory, Carnegie Institution of Washington

Compared with meteorites, Earth and the other inner planets of our solar system contain relatively low amounts of water and volatile elements, which were not abundant in the inner solar system during planet formation. The even lower quantites of these volatile elements found on the Moon has long been claimed as evidence that it must have formed following a high-temperature, catastrophic giant impact. But this new research shows that aspects of this theory must be reevaluated.  The study also provides new momentum for returning similar samples from other planetary bodies in the solar system.

“Water plays a critical role in determining the tectonic behavior of planetary surfaces, the melting point of planetary interiors, and the location and eruptive style of planetary volcanoes,” said Hauri. “These kinds of volcanic deposits have been mapped on Mars and Venus, and even now are being mapped on Mercury by the MESSENGER mission; it is no longer a mystery where these deposits are and where to go to bring them back to Earth.  We can conceive of no sample type that would be more important to return to Earth than these volcanic glass samples ejected by explosive volcanism, which provide critical information on the water content of planetary interiors.”

Three years ago the same team, in a study led by Saal, reported the first evidence for the presence of water in lunar volcanic glasses and applied magma degassing models to estimate how much water was originally in the magmas before eruption.  Building on that study, Weinreich, a Brown University undergraduate, found the melt inclusions, allowing the team to measure the pre-eruption concentration of water in the magma and estimate the amount of water in the Moon's interior.

“The bottom line,” said Saal, “is that in 2008, we said the primitive water content in the lunar magmas should be similar to lavas coming from the Earth's depleted upper mantle. Now, we have proven that is indeed the case.”

The study also puts a new twist on the origin of water ice detected in craters at the lunar poles by several recent NASA missions.  The ice has been attributed to comet and meteoroid impacts, but it is possible that some of this ice could have come from the water released by past eruptions of lunar magmas.

These findings should also be taken into account when analyzing samples from other planetary bodies in our solar system. The paper’s authors say these results show that their method of analysis is the only way to accurately and directly determine the water content of a planet’s interior.

The research was supported by CIW, NASA’s LASER and Cosmochemistry programs, the NASA Lunar Science Institute, and the NASA Astrobiology Institute.