ASTRONOMERS HAVE identified five big craters at the south pole of the Moon, which may contain large amounts of frozen water that could be used for a future lunar base. The craters are up to 32 miles across and 1.5 miles deep and have been in perpetual shadow for many millions of years - preventing evaporation of any ice from the crater floor.

Scientists from the Jet Propulsion Laboratory in California and Cornell University in New York identified the craters, using radar images collected from Nasa's Deep Space Network. They built up the first detailed, 3D images of the hidden craters at the lunar poles to reveal five that are substantial enough to harbour water in the form of ice.

The news will help Nasa to decide whether to crash-land its nearly defunct Lunar Prospector spacecraft on to one of the sites, so that it can provide definitive evidence that the Moon has its own water.

Astronomers hope that a controlled, high-speed dive into one of the craters, scheduled for next month, will evaporate some of the ice into a plume of water vapour, which could then be detected by instruments on Earth. The latest radar images, published in the journal Science, were made by bouncing radar waves off the Moon and collecting the reflections at two radio telescopes on Earth.

Lunar Prospector raised the possibility of there being water in the form of ice on the Moon last year when it detected hydrogen at the north and south poles.

Dr Jean-Luc Margot, a former researcher at Cornell, said the lunar orbiter has detected significant amounts of hydrogen in one of the large craters, which is informally named the Mawson crater. "The argument for targeting that particular crater is that it is both in permanent shadow, as shown by our radar data, and also has a high hydrogen abundance, as shown by new Lunar Prospector data. This makes it a prime candidate for water ice deposits," he said.

Scientists believe that huge ice-bearing comets crashed into the Moon in the past and that some of the frozen water might have remained intact if it has been kept in permanent shadow. Temperatures in these "cold traps" do not rise above minus 170C.

If there are significant amounts of water on the Moon they could be used as a source of hydrogen fuel and oxygen for a future lunar base. Some scientists have proposed that a lunar base could be used as a stepping stone for a manned mission to Mars.