The next question is just how much. Were the ice present in the form of a solid sheet or chunks, the dips in the numbers of epithermal neutrons would be much greater. The present data suggest that the ice is in the form of small crystals, comprising about 0.3 percent to 1 percent of the moon's rocky soil. At such concentrations, a cubic yard of soil would contain as much as five gallons of water.
The key issue is how deeply the water extends into the lunar regolith. The present estimate is based on the depth to which the neutron spectrometer's signal can penetrate--about a foot and a half. But James Arnold of the University of California at San Diego estimates the amount of lunar regolith that could have been "gardened" by all impacts in the past two billion years extends to a depth of about 6.5 feet. If he is correct, Lunar Prospector's estimate of water ice would have to be increased by a factor of up to four, to the range of 44 million to 1.3 billion tons.
Still, the amount of water in current estimates represents a potential bonanza. "Our data are consistent with the presence of water ice across a significant number of craters," said William C. Feldman, co-investigator and spectrometer specialist at the Los Alamos National Laboratory. "There is enough there to support a modest amount of colonization for centuries--we have paved the way for future missions."