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 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.
The idea of looking for water in lunar apatite isn't new, Boyce notes. "Charles B. Sclar and Jon F. Bauer, geoscientists at Lehigh University, first noted that something was missing from the results of chemical analyses of apatite in 1975," he says. "Now, 35 years later, we have quantitative measurements -- and it turns out, they were right. The missing piece was OH."
The Caltech team analyzed the lunar apatite for hydrogen, sulfur, and chlorine using an ion microprobe, which is capable of analyzing mineral grains with sizes much smaller than the width of a human hair. This instrument fires a focused beam of high-energy ions at the sample surface, sputtering away target atoms that are collected and then analyzed in a mass spectrometer. Ion microprobe measurements demonstrated that in terms of its hydrogen, sulfur, and chlorine contents, the lunar apatite in this sample is indistinguishable from apatites from terrestrial volcanic rocks.
"We realized that the moon and the earth were able to make the same kind of apatite, relatively rich in hydrogen, sulfur and chlorine," Boyce says.
Does that mean the moon is as awash in water as our planet? Almost certainly not, say the scientists. In fact, the amount of water the moon must contain to be capable of generating hydroxyl-rich apatite remains an open question. After all, it's hard to scale up the amount of water found in the apatite -- 1600 parts per million or 0.16 percent by weight -- to determine just how much water there is on the lunar landscape. The apatite that was studied is not abundant, and is formed by processes that tend to concentrate hydrogen to much higher levels than are present in its host rocks or the moon as a whole.
"There's more water on the moon than people suspected," says Eiler, "but there's still likely orders of magnitude less than there is on the earth."
Nonetheless, the finding is significant for what it implies about our moon's composition and its history. "These findings tell us that the geological processes on the moon are capable of creating at least one hydrous mineral," Eiler says. "Recent spectroscopic observations of the moon showed that hydrogen is present on its surface, maybe even as water ice. But that could be a thin veneer, possibly hydrogen brought to the moon's surface by comets or solar wind. Our findings show that hydrogen is also part of the rock record of the moon, and has been since early in its history."
Beyond that, Eiler continues, "it's all a great big question mark. We don't know whether these were igneous processes," -- in which rocks are formed by solidification of molten lava -- "or metamorphic" -- in which minerals re-crystallize or change in change in chemistry without melting. "They're both on the table as possible players."
In addition to Boyce, Eiler, Rossman, and Taylor, other authors on the Nature paper, "Lunar apatite with terrestrial volatile abundances," include Research Assistant Professor Yang Liu from the University of Tennessee in Knoxville; Edward Stolper, Caltech's William E. Leonhard Professor of Geology, and Yunbin Guan, manager of Caltech's ion microprobe laboratory.
Their work was funded by grants from NASA's Cosmochemistry Program, the National Science Foundation, and the Gordon and Betty Moore Foundation.
- Jeremy W. Boyce, Yang Liu, George R. Rossman, Yunbin Guan, John M. Eiler, Edward M. Stolper & Lawrence A. Taylor. Lunar apatite with terrestrial volatile abundances. Nature, 2010; 466 (7305): 466 DOI: 10.1038/nature09274
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