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NASA moon impact blasted into material that’s 5% water

The results are in from NASA's LCROSS lunar impact mission, and they show …

About a year ago, NASA held a press conference to describe the preliminary results of its analysis of the LCROSS experiment, in which a spent rocket stage plowed into the Cabeus crater at the Moon's south pole. As the impact took place, there was no visible debris plume, but NASA captured it in the UV and IR portions of the spectrum, and was able to announce that there were "significant quantities" of water seen in the debris. Now that the dust has long since settled, a total of six papers and a perspective that describe the results have appeared in Science.

One of the papers describes the impact site in the Cabeus Crater, and other polar craters like it, based on thermal data from the Lunar Reconnaissance Orbiter (LRO). The shaded areas near the craters' walls, which never receive sunlight, should in theory be exceptionally cold, with the only heat coming from radiation by the opposing, sunlit crater wall. An LRO instrument confirms that this is actually the case, producing estimates that these "cold traps" are hovering only 38K above absolute zero.

"Temperatures in the Moon’s larger cold traps are closer to those expected for the poles of Pluto than for Earth’s closest neighbor," the authors conclude.

All of which made Cabeus a great choice to crash a rocket into, since temperatures that cold mean that relatively volatile compounds, including water, don't sublimate (go directly from solid to gaseous form) at appreciable rates, even if no atmosphere is present.

And, as NASA first announced, the infrared instrument that followed the rocket in picked up evidence of water in the impact plume. The new data analysis now goes a long way towards telling us how much is there by estimating both the total size of the plume and the percentage of water in it.

As a separate paper describes it, the debris plume actually became visible in stages. First up was mineral material, the dust and rocks blasted out of the crater by the impact itself. By 30 seconds after impact, however, this material had started to fall back towards the lunar surface, after which the spectrum of the debris cloud became dominated by materials in gaseous form, such as vaporized water. The instrument was also able to estimate the total size of the cloud, which was essential for figuring out the volume of material blasted free by the impact.

Running the numbers, the scientists concluded that the surface material in Cabeus is between five and six percent water, with most of that in the form of pure ice covered by a protective layer of dust. The results confirm that the signal doesn't arise from water incorporated into a mineral, or from compounds like aluminum hydroxide. All told, several hundred kilograms of water were liberated during the LCROSS impact.

Water wasn't the only volatile compound that showed up in the data, however. Signals corresponded to various organic species, including carbon dioxide, carbon monoxide, and various nitrogen and sulfur bonds. And, in a bit of a surprise, hydrogen seems to have been present in its molecular form (a neutral H2 molecule). Presumably, it's absorbed onto the surface of a mineral in the lunar material. Roughly 140kg of hydrogen was blasted free by the impact—by weight, that's more than the magnesium present.

The infrared instrument also may have picked up signs of sodium and silver, while a UV instrument on the LRO mission found signs of mercury, calcium, and magnesium.

All in all, these are pretty significant results. Some of the earlier hints of water on the Moon were consistent with the water being produced as protons are delivered to the Moon's surface by the solar wind. The significant quantities of other volatiles seem to suggest something more complex is going on; "Going by the evolving spectra, the impact released a variety of volatiles besides water, which suggests a variety of possible sources besides solar wind implantation," the authors of one paper conclude. These possible sources would include delivery of material via comets and a more active surface chemistry than anyone expected.

A perspective that describes all of these results suggests that we probably won't learn much more about how these materials got there without going to see them in their native state ourselves. And that's looking a bit more manageable. Assuming we can keep the water free of the mercury that's also present and deal with Pluto-level temperatures, then there should be useful quantities of water present in Cabeus and other cold sinks.

Science, 2010. DOI: 10.1126/science.330.6003.434  (About DOIs).

(The DOI link is for the perspective, which contains references for the other papers.)

Listing image by NASA

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