The most exacting analysis yet of the moon's mysterious polar craters found no sign of the vast expanses of ice that scientists had hoped future lunar colonists could someday mine for precious, life-sustaining water.
The findings do not mean there is no ice in the permanently shaded craters. But if there is ice, it is probably mixed into the lunar soil in widely scattered flecks or in thin layers.
"It certainly would have been nice to find some sort of lunar skating rink, or thick layers of ice, but it looks like it's just not there," said Bruce Campbell of the Smithsonian Institution's Center for Earth and Planetary Studies.
He and colleagues at Cornell University used the mammoth radar dish at Puerto Rico's Arecibo Observatory to bounce radio waves off the craters. They probed more deeply than ever before into the craters' floors -- as far as 20 feet down in the soil.
Like earlier Earth-based radar imaging, the analysis showed no sign of thick ice layers. The findings appear in the current issue of Nature.
Campbell said the work supports the idea that any ice in the moon's polar regions is in thin layers or scattered crystals. That means that moon colonists would need equipment to either sort ice particles from the soil or heat up the crater floors and collect the water vapor.
Alan Binder of the Lunar Research Institute in Tucson said the only way to determine how much ice is on the moon is to send a human or a robot.