WASHINGTON, D.C.--A spacecraft has found "significant deposits of water ice at both poles of the moon," scientists announced at a press conference here today after analyzing data from the first mission to explore the moon in 25 years. The discovery by the dirt-cheap NASA mission, called Lunar Prospector, promises a practically limitless supply of water that could drastically reduce the costs of lunar colonization.
Planetary scientists have speculated on the possibility of water near the lunar poles for decades despite the moon's obvious dryness, because icy comets have presumably been pelting the moon for billions of years. The blistering heat of the sun--which cooks the moon's surface to about 120 degrees Celsius--should have boiled off much of the cometary water. But in theory, a tiny fraction could have been secreted away in the interiors of craters near the poles that are permanently shaded by their rims. Radar probing by a Defense Department spacecraft seemed to detect polar ice in 1996 (Science, 29 November 1996, p. 1495 ), but a subsequent Earth-based radar study contradicted it (Science, 6 June 1997, p. 1527 ).
Lunar Prospector went looking for water in mid-January using an instrument new to planetary exploration. The neutron spectrometer on board, operated by William Feldman of the Los Alamos National Laboratory in New Mexico, recorded neutrons produced by high-energy cosmic rays striking the lunar surface. If these fast-moving neutrons bounce off a hydrogen atom--two of which are found in every water molecule--before ricocheting up to the spacecraft, they will slow down and reveal the presence of water.
After a month of passes over the lunar poles, "we're certain water is there," says Alan Binder of the Lunar Research Institute in Gilroy, California, principal investigator of Lunar Prospector. "The uncertainty is in how much." Something like 0.5% to 1% of the lunar soil near the poles appears to be fine particles of ice, says Binder, which at 5 to 20 liters of water per cubic meter of soil might add up to 10 million to 300 million metric tons of water. "That's a significant quantity," he says. A little heating after being mined could drive water vapor off for collection for drinking, farming, and splitting into hydrogen and oxygen, the perfect rocket fuels. This "moonshine," says Feldman, could allow "a modest amount of colonization for centuries."