Craft finds evidence of ice on the moon.

A spacecraft has gathered the best evidence yet that ice lies deep within craters at the moon's north and south poles. Data from NASA's Lunar Prospector, launched 2 months ago, suggest that the sunless craters contain frozen water--perhaps 300 million metric tons.

That doesn't mean Tara Lipinski should grab her skates and head for the moon. The ice is probably in the form of frost mixed with lunar soil. By weight, it makes up only about 1 percent of the floor material of lunar craters that lie in permanent shadow, William Feldman of the Los Alamos (N.M.) National Laboratory reported last week at a press briefing at NASA's Ames Research Center in Mountain View, Calif.

He and team leader Alan Binder of the Lunar Research Institute in Gilroy, Calif., deduced the presence of water from data gathered by Prospector's neutron spectrometer. Cosmic rays hitting the moon generate energetic neutrons, which rattle around the lunar surface before escaping into space. These neutrons lose only a little energy when they bounce off heavy nuclei in the soil, but they give up much more in collisions with hydrogen nuclei, whose masses are close to that of the neutrons.

The spectrometer recorded a dip in the number of energetic neutrons as Prospector passed over the poles, indicating that these regions are slightly enriched in hydrogen. The hydrogen is almost certainly tied up in water, the Prospector team asserts. Feldman notes that the poles are not chilly enough to freeze two other hydrogen-rich gases--ammonia and methane--that are likely to be present. Although water is abundant in the cosmos, he admits that inferring its presence on the moon "is a leap of faith."

That's a leap many planetary scientists are willing to make. In 1994, the Clementine craft bounced radio waves off the moon and found tentative evidence of frozen water inside polar craters (SN: 6/11/94, p. 383). "I find the Prospector results convincing and fascinating," says Clementine researcher Paul D. Spudis of the Lunar and Planetary Institute in Houston.

Binder touts the finding as a boon for space travelers. "For the first time, when we go to a planetary body ... you can fuel up." Other researchers argue that gaining access to the ice, whether for hydrogen fuel or drinking water, might be more trouble than it's worth. "Digging out that ice is going to be cold, dark, and difficult work," notes Steven W. Squyres of Cornell University.

Researchers believe that the moon was hot and dry when it formed, but over billions of years it acquired water from comets pelting it. Because the sun makes a shallow angle at the moon's poles, the bottoms of craters there never see sunlight and are an ideal place to trap frozen water, Binder says. The amount of ice may indicate the frequency of comet hits and how long the poles have been in their present orientation.

Also at the briefing, Alex Konopliv of NASA's Jet Propulsion Laboratory in Pasadena, Calif., unveiled a new gravity map. It provides a peek beneath the lunar surface and makes possible more accurate estimates of the fuel required to orbit the moon.