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.
Paul Lucey, a planetary scientist at the University of Hawaii at Manoa who wrote a commentary in Science to accompany the three spectroscopic studies, says that he found the results "pretty stunning."
"I was on vacation when I read the first paper, and I used colorful language when I read it," Lucey says. "I was amazed." At the same time, he says, it is not certain that the spectra show both water and hydroxyl. "We see OH or H2O," he says. "I believe further analysis of the data that now exists will probably allow distinguishing between those two cases."
On the other hand, Lucey notes, the Deep Impact study "is suggesting that the signature is changing with temperature or time, so that suggests to me that water is more likely, just because OH binds so tightly to minerals and is not going to be very mobile." In some places, such as near the equator, where daytime temperatures are high, Deep Impact saw the signal dissipating by the time the sun was directly overhead, returning when cooler temperatures arrived in the lunar evening.
One explanation for that phenomenon is that a stream of charged hydrogen atoms in the solar wind could react with oxygen-bearing lunar minerals to produce water at the surface. That process would explain the steady, fast-acting replenishment seen in the data after sunlight has dissociated the water molecules.
Sunshine notes that in her view it is not so much a question of whether hydroxyl or water is present but how much each contributes to the spectral signature. "The water and hydroxyl sort of mix, and it's more complicated to know what is uniquely water versus uniquely OH," she says. "However, we are seeing changes as this water loss happens, and we see changes in the different parts of the absorption feature, so we're seeing different species coming in and out. The simplest explanation for that is certainly that you have water being lost. OH is a much stronger bond; it's harder to get rid of."
Pieters says the data from the three papers, taken together, settle the water question. "Basically, the bottom line, if you read all three of them, is there is no question that water and hydroxyl exist on the surficial upper layers of the moon," she says.
So why has this widespread surface phenomenon never been uncovered before, especially given that its discovery relies in part on 10-year-old data? "I think it's just one of those funny science sociological phenomena that it just didn't occur to take the measurement," Lucey says.
Sunshine says that the focus on polar traps, such as that sought by LCROSS, tended to dominate the search for water on the lunar surface. "Everybody tends to think about this in terms of polar ice caps and skating rinks and lakes, and we're talking about molecules," she says. "It's a real shift in the way people think about water on the moon."