The thick ice sheets many astronomers thought were hidden in some of the Moon鈥檚 craters almost certainly do not exist, according to observations from the world鈥檚 biggest radar.
The 300-metre Arecibo dish in Puerto Rico was used to bounce radio waves into dark craters at the Moon鈥檚 north and south poles. Sunlight never reaches the bottom of these craters, meaning ice could survive there.
In 1994, the US spacecraft Clementine probed the craters in the same way from lunar orbit. It detected strong echoes that could have been produced by thick sheets of ice. Four years later, NASA鈥檚 Lunar Prospector spacecraft probed the craters with a neutron spectrometer. This led to a positive detection of hydrogen, and thus water, at the lunar poles.
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Putting the two observations together, scientists concluded that millions of tonnes of water had formed thick ice sheets in the permanently shadowed lunar craters.
But the new Arecibo investigation has come to a very different conclusion. Team leader Bruce Campbell, at the Smithsonian Institution, Washington DC, told New Scientist: 鈥淭here are no thick ice deposits in the areas we observed.鈥� The researchers surveyed 20 per cent of the Moon鈥檚 shadowed regions.
Tilted walls
One crater, called Shackleton, did return a strong echo, mimicking the Clementine data. But so did another crater that is not permanently shadowed, meaning it could not be caused by ice.
鈥淲e think that the rough, tilted walls of those craters were producing the strong reflections, not ice,鈥� Campbell explains.
The new data does not rule out ice altogether. The experiment would only detect ice sheets more than a metre thick. Any thinner deposits, or small ice crystals distributed in the lunar dust, would have remained undetected.
Moon base
Nonetheless, it is bad news for proponents of a Moon base, who have argued that readily available water would make such a base easier to set-up.
Campbell estimates that, at best, a cubic kilometre of lunar soil would have to be processed to extract just a cubic metre of water, making it a very difficult and expensive endeavour.
An improved estimate of how much ice is on the Moon may be available in 2005, when the European Space Agency鈥檚 SMART-1 arrives. It is carrying an infrared spectrometer that will look straight down into the shadowy craters for ice deposits.
Beyond that, Campbell believes that the only way to settle the debate is to land a spacecraft. He is part of a team designing such a mission, called Polar Night. The team plans to submit the proposal to NASA in 2004.
Journal reference: Nature (vol 426, p 137)