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."