Lunar water brings portions of Moon's origin story into question

The Moon has much more water than previously thought, a team of scientists led by Carnegie's Erik Hauri has discovered. Their research, published May 26 in Science Express, shows that inclusions of magma trapped within crystals collected during the Apollo 17 mission contain 100 times more water than earlier measurements. These results could markedly change the prevailing theory about the Moon's origin. 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 beads—orange in appearance because of their high titanium content—which contained crystal-hosted melt inclusions. These inclusions were prevented from losing the water within 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 science team. The paper's authors are Hauri; Thomas Weinreich, Alberto Saal and Malcolm Rutherford from Brown University; and Van Orman.

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, a geochemist with Carnegie's Department of Terrestrial Magnetism (DTM). "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 have been mapped not only on the Moon but throughout the inner solar system."

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 the water content in 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.

Source: Carnegie Institution