Shortly after NASA's LCROSS probe slammed into a crater in the polar region of the Moon, the Agency held a press conference to announce that it had obtained significant amounts of data from the collision. Unfortunately, to the frustration of many present, it wasn't ready to interpret that data. That reticence ended today, as NASA held a press conference in which it announced that the data contains unambiguous evidence of water, present in what it termed "significant quantities." But the signal from water isn't the only one lurking in the data, and NASA is remaining coy about what the other signals indicate.
Back in October, the LCROSS mission sent two objects crashing into the Cabeus crater, which has an interior that is permanently shadowed due to its location in the Moon's polar region. The first was a heavy booster rocket; its collision was imaged by instruments on the actual LCROSS probe, which followed it into the crater a few minutes later. Its collision was tracked by Earth- and space-based instruments.
Initially, there was a degree of disappointment about the lack of a dramatic plume of debris that accompanied the impact. But NASA reassured everyone that the relevant instruments worked well, and that there was a wealth of data available to analyze. That set off further frustration, as none of the NASA scientists were willing to speculate what that data said about the composition of the material in the crater, since they weren't confident about their preliminary analysis.
After a few weeks of looking things over, they're apparently feeling a lot more confident. Both the UV/visible and infrared spectrometers returned detailed data, both in terms of wavelength and time post-impact, and they clearly showed a complex pattern of emission and absorption. The question the scientists had to grapple with is whether any single combination of chemicals could produce that spectrum, or if the spectrum was compatible with a number of scenarios.
According to Anthony Colaprete, LCROSS' scientific lead, they've since eliminated any alternative they can think of—the data contains unambiguous evidence of water. He showed the infrared spectral data, and contrasted that with the null case, blackbody radiation. In at least two areas of the spectrum, a curve produced by modeling blackbody radiation plus water absorption matched the curve nearly exactly. The UV/visible data showed emission lines from hydroxyl bonds, which also suggest water.
Based on the strength of the signal and size of the plume, NASA estimates that about 100kg of water were kicked up in the impact plume, which Colaprete says is enough to indicate that there were actual ice deposits within the crater. In a response to a question, he suggested that this would make it wetter than South America's Atacama Desert.
But Colaprete couldn't resist a further tease, saying that other features in the spectra hinted at a variety of additional chemicals. "This goes beyond the water," he said, but declined to discuss any of this additional information, suggesting that it was in the same state as the water findings were shortly after the impact—they still haven't eliminated enough possible explanations, so they can't talk about it with any confidence.
Ultimately, that data may tell us very significant things about the status and behavior of the water. Papers that argued for lunar water had suggested different explanations for the origin of that water, such as cometary delivery and capture of the solar wind. They also hinted that there might be an active cycling of the water, driven by the changes in light that accompany the long solar day. The new data, should they contain chemicals typical of comets, could help inform the debate over what's happening on the moon.
In any case, it's important to emphasize the mental U-turn the scientific community has to make here. For decades following the Apollo missions, scientists were operating based on the evidence returned from the Moon, which suggested a completely dry environment. The three papers that first suggested a revision to those conclusions were only published in September, and LCROSS data was only obtained in October. It will undoubtedly take a while for the full implications of these findings to sink in.