It's localized in spots near the poles, and it has to be buried, about half a meter or so. As we've gotten more data we've found that it's not spread out as we first assumed, but concentrated. When they presented their initial results in March, the scientists said the water was likely in the form of a fine frost spread through the lunar soil. Further data analysis now allows the possibility of deposits of solid ice, Feldman said. Feldman currently estimates there may be as much as three billion metric tons of water ice at each of the poles, with 15 percent more at the north pole than at the south pole.
Scientists assume comets carry the water ice to the moon.
The comets basically vaporize on impact, and the water molecules migrate to the permanently shaded regions at the poles. These regions are so cold that once a water molecule enters them it gets stuck. Alan Binder of the Lunar Research Institute is the principal investigator for the mission. Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.