WASHINGTON, D.C.--Planetary scientists announced at a press conference this afternoon that they have found strong signs of ice secreted in a dark cranny near the moon's south pole. If astronauts could mine such icy deposits, they would have the essentials--air to breathe, water to drink, and fuel for their rockets--for lunar colonization and a steppingstone to Mars and other planets. Details of the finding are published in a Report  in the current issue of Science.
Scientists have long imagined ice hidden from the moon's searing daytime temperatures in pockets of permanent shadow near the poles where the sun never reaches. But it took the little Clementine spacecraft, a low-cost mission of the Department of Defense and NASA, to shine a radar "flashlight" into the permanent darkness of one corner of a great impact crater and get a return flash of the sort that ice can make. Known ice lodes have produced similar flashes on Jupiter's moons, the south polar cap of Mars, and the Greenland ice sheet. "It's not an ice rink on the moon," says lunar geologist Paul Spudis of the Lunar and Planetary Institute in Houston, but "we think we've discovered ice. The significance of this for future exploration of the solar system is very profound."
The potential significance may be profound, but the technology that yielded signs of ice is not. While orbiting the moon, Clementine bounced its 6-watt communications signal off the surface to be picked up by big antennas back on Earth. On three passes--two over the north pole and one near the more heavily shadowed south pole--nothing extraordinary happened. But on a pass that included an area in permanent shadow near the south pole, the reflected signal brightened just as expected if it had bounced around within subsurface ice to produce a "road-sign reflector effect," says team member Stuart Nozette. The reflected signal's polarization appeared as if it had reflected from radar-permeable ice rather than rock.
The Clementine team envisions one or more patches of buried ice dirtied by lunar soil or even soil laced with ice crystals, covering more than 100 square kilometers. Presumably comets crashing into the moon over the eons left water vapor that condensed in permanent shadow at 40 degrees above absolute zero and never escaped.
Some planetary radar specialists, however, believe the ice may be a figment of the imagination. The signal "does all the right things," says Gordon Pettengill of the Massachusetts Institute of Technology, "but it's only one pass. It's very suggestive, but I wouldn't give it more than a 30% chance of being real." Conveniently enough, NASA's Lunar Prospector spacecraft, due for launch next October, will be carrying a neutron spectrometer that can sniff out ice in the deepest of shadows.