The Year in Science: Astronomy 1997

How Dry the Moon?

By Shanti Menon|Thursday, January 01, 1998
Is there ice on the moon? When the spacecraft Clementine bounced a radar beam off the moon’s poles in 1994, the echoes from some of the deep, permanently shadowed craters did seem to show the signature of ice—left over, perhaps, from long-ago cometary impacts. But last June, astronomers using the giant radar telescope at the Arecibo Observatory in Puerto Rico said they could find no sign of that ice.

Donald Campbell of Cornell and his colleagues made a radar map of the moon’s poles that showed features as small as 400 feet across, a thousand times sharper than Clementine’s view. Only a few small spots showed an icelike radar signature. And when Campbell compared the map with photographs of the moon, most of those spots turned out to be in sunlight and thus would have been too warm to hold ice. Campbell thinks the signal came from rough surfaces inside the craters, which distort radar in a manner similar to ice. There are still some deep craters that neither Clementine nor the Arecibo telescope could get a good angle on, he says, but if Clementine had seen anything, we should have seen it, too. My feeling is that if there were any ice on the moon, there would be more than a puddle or two hiding in the corners.

The Clementine researchers, however, stand by their results—and they’re counting on another spacecraft, the Lunar Prospector, to settle the matter. Scheduled for launch in January, that probe will map the chemical composition of the moon from an orbit over the poles.
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