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Scientists dimmed hopes of finding large amounts of water on the dark side of the moon today with a new study showing minimal evidence of deposits in craters.
Data from the Clementine and Lunar Prospector space missions in the 1990s had suggested the possibility of significant water supplies, perhaps vital to support life and human colonies on the moon, stored in craters near the poles.
But after conducting a radar survey of craters constantly in shadow at the lunar poles, Bruce Campbell and his Smithsonian Institution colleagues said they did not uncover any evidence of thick ice deposits.
"Any lunar ice present within the regions visible to the Arecibo radar must therefore be in the form of distributed grain or thin layers," Campbell stated in a report in the science journal Nature.
Wavelengths from the radio telescope at the Arecibo Observatory in Puerto Rico can penetrate deep into the craters, but they did not produce the strong echoes indicative of large chunks of ice.
Weaker echoes suggest any water there would likely be embedded in rock.
If there is water, it would probably have been deposited when comets, composed mainly of ice, collided with the moon over billions of years.
The scientists believe most of the water would have evaporated into space but some may have collected in the shadowed craters at the lunar poles.
Water on the moon could be used for rocket fuel and to establish a refueling station there for solar exploration.
Campbell's team believes the findings suggest either fewer comets collided with the moon than with other planets, or there was a rapid loss of ice on the lunar surface.