Slamming a spent rocket booster into the frigid, inky shadow of a lunar crater last month sent up a plume of dust laced with water, NASA scientists reported in a press conference today. Observers on Earth were denied a view of the fireworks in October, but "we found water, a significant amount of water," said LCROSS (Lunar Crater Observation and Sensing Satellite) mission principal investigator Anthony Colaprete of NASA's Ames Research Center in Mountain View, California.
After several decades of controversy, scientists now know that over billions of years, water from who knows where--impacting comets or perhaps the solar wind--can collect in some of the coldest places in the solar system. Whether the predicted amount of water is enough to sustain future astronauts--as either sustenance or rocket fuel--remains to be seen, however.
The LCROSS mission worked to perfection, with the exception of the show that unfolded on Earth. Before the impact, NASA scientists had predicted that ground-based observers, even amateurs, would see the plume in the gap between two mountains. As it turned out, Colaprete said, the impact's plume of debris "was as bright as thought, but it was behind a hill" because the debris did not rise as high as impact modeling had suggested.
The heavily instrumented LCROSS spacecraft, however, had a fine view of the rocket booster's impact and aftermath as it sped to its own impact 4 minutes later. LCROSS instruments delivered a "good, strong detection" of water, Colaprete said. At infrared and ultraviolet wavelengths, they gave clear indications of water vapor, water ice, and hydroxyl ions produced when sunlight splits water molecules.
All told, LCROSS detected about 100 kilograms of water, Colaprete said. It came from a 20-meter-wide crater maybe 3 meters deep, but he declined to guess how abundant water ice had been beneath the impact site. Team members must still calculate what portion of subsurface ice actually rose into view and could have been measured, Colaprete noted. "It would probably be safe to say it's wetter than the Atacama Desert," the driest place on Earth, he said. Some remote sensing had suggested about 1% water ice by volume in the upper 3 meters, which was regarded as a substantial amount. Impact modeler and LCROSS team member David Goldstein of the University of Texas, Austin, says 1% "is not inconsistent with what's been observed. I haven't convinced myself yet whether it's 0.1% or 10%. I think we'll work that out."
Whatever the amount, the principle of cold-trapping water in permanently shadowed craters--as had been demonstrated for Mercury using radar--is now firmly established for the moon. The LCROSS results give only an inkling of where the water might have come from. Colaprete reported that spectra hint at the presence of volatile compounds besides water, such as carbon dioxide, methane, sulfur dioxide, and methanol, just the sort of compounds found in comets and ice-rich asteroids. So the moon may have retained a tiny bit of the objects that have pummeled it for eons.