WATER FROM WIND: In this diagram, which models one possible explanation for water molecules that have been detected at low density across the lunar surface, hydrogen ions in the solar wind bombard the moon's sunward side and react with oxygen-bearing compounds to form water. At lunar noon, when the temperature reaches its peak, sunlight can break water molecules apart, but in cooler hours the solar wind contributes to their re-formation. Image: Image courtesy of University of Maryland/F. Merlin/McREL
A hotly anticipated experiment will test the theory next month that the moon's permanently shadowed polar craters harbor pockets of water ice. A NASA spacecraft called the Lunar Crater Observation and Sensing Satellite (LCROSS) will perform a two-stage bombardment of a south polar crater to see what rises up in the ensuing debris plume.
Now, just two weeks before LCROSS's scheduled barrage, comes a suite of evidence that the moon indeed hosts water. But the new studies point to a different sort of deposit than the concentrated ice supply LCROSS seeks—they indicate that water exists diffusely across the moon as molecules clinging to the surface in low concentrations. What is more, there may be a water cycle in which the molecule is broken down and reformulated over the span of a lunar daytime (about two Earth weeks long).
A trio of papers published online this week in Science, each using spectroscopic data collected by a different spacecraft, find light absorptions characteristic of water or hydroxyl (OH) molecules or both. And the papers' authors say that the scenario in which both molecules appear across the lunar surface is the most plausible explanation for their data.
Water and hydroxyl are related molecular species and have similar spectroscopic signatures—the wavelengths characteristic of each reside nearby in the infrared portion of the electromagnetic spectrum. The three-micron absorption band indicative of water appeared broadly across the lunar surface in spectrometric data taken by the Moon Mineralogy Mapper (M3), an instrument that circled the moon aboard India's Chandrayaan 1 spacecraft until the orbiter's mission ended prematurely last month.
At first, the instrument's minders were confounded, figuring that something on M3 had gone awry. "The first reaction that I think all of us had was, this is ridiculous," says Carle Pieters, a planetary scientist at Brown University and principal investigator for M3. The team was finding evidence for water not in permanently shadowed craters but on the sunlit portions of the moon, which just did not add up.
"We spent months going through our data, trying to find what went wrong," she recalls. "What is it that gives us this signature that we can't get rid of?" Unable to troubleshoot the odd result, Pieters's group turned to a second, and then a third, independent observation.
Roger Clark, a U.S. Geological Survey spectroscopist on the M3 team, reanalyzed archival data from the Cassini spacecraft, now exploring Saturn and its satellites, taken during a 1999 flyby of the moon. The Cassini data agreed with the finding that water appears to be widespread across the lunar surface.
Yet more confirmation came from a timely flyby of the Deep Impact probe, en route to a cometary rendezvous in 2010. In June the spacecraft swung past the moon, and its spectrometer put the lunar water theory to the test—a test that went swimmingly well.
"In the Deep Impact data, we have very strong evidence that [water is] everywhere," says planetary geologist Jessica Sunshine, a senior research scientist at the University of Maryland, College Park, who works on both the M3 instrument and the Deep Impact mission. "There is no place on the moon that we don't see this." She notes that the water appears to hug the lunar surface—reaching depths measured in millimeters or even hundreds of microns—and that the local abundance in a typical area appears quite low. "We're still talking about amounts of water that are less than the hottest desert you could think of here," Sunshine says.