Caltech Team Finds Evidence of Water In Moon Minerals

This is a photomicrograph of a thin section of lunar sample 14053. Credit: Larry Taylor/University of Tennessee

That dry, dusty moon overhead? Seems it isn't quite as dry as it's long been thought to be. Although you won't find oceans, lakes, or even a shallow puddle on its surface, a team of geologists at the California Institute of Technology (Caltech), working with colleagues at the University of Tennessee, has found structurally bound hydroxyl groups (i.e., water) in a mineral in a lunar rock returned to Earth by the Apollo program.

Their findings are detailed in this week's issue of the journal Nature.

"The moon, which has generally been thought to be devoid of hydrous materials, has water," says John Eiler, the Robert P. Sharp Professor of Geology and professor of geochemistry at Caltech, and a coauthor on the paper.

"The fact that we were able to quantitatively measure significant amounts of water in a lunar mineral is truly surprising," adds lead author Jeremy Boyce, a visitor in geochemistry at Caltech, and a research scientist at the University of California, Los Angeles.

The team found the water in a calcium phosphate mineral, apatite, within a basalt collected from the moon's surface by the Apollo 14 astronauts.

To be precise, they didn't find "water"-the molecule H2O. Rather, they found hydrogen in the form of a hydroxyl anion, OH-, bound in the apatite mineral lattice.

"Hydroxide is a close chemical relative of water," explains coauthor George Rossman, Caltech's Eleanor and John R. McMillan Professor of Mineralogy. "If you heat up the apatite, the hydroxyl ions will 'decompose' and come out as water."

The lunar basalt sample in which the hydrogen was found had been collected by the Apollo 14 moon mission in 1971; the idea to focus the search for water on this particular sample was promoted by Larry Taylor, a professor at the University of Tennessee in Knoxville, who sent the samples to the Caltech scientists last year.

"The moon has been considered to be bone dry ever since the return of the first Apollo rocks," Taylor notes. However, there are lunar volcanic deposits interpreted as having been erupted by expanding vapor. Although carbon dioxide and sulfur gases have generally been thought to dominate the expanding vapor, recent evidence from the study of the these deposits has suggested that water could also play a role in powering lunar volcanic eruptions. The discovery of hydroxyl in apatite from lunar volcanic rocks is consistent with this suggestion.