A surprising amount of water has been found to exist in the Moon's soil.

Data from three spacecraft, including India's Chandrayaan probe, shows that very fine films of H20 coat the particles that make up the lunar dirt.

The quantity is tiny but could become a useful resource for astronauts wishing to live on the Moon, scientists say.

"If you had a cubic metre of lunar soil, you could squeeze it and get out a litre of water," explained US researcher Larry Taylor.

The rock and soil samples returned by the Apollo missions were found to be ever so slightly "damp" when examined in the laboratory, but scientists could never rule out the possibility that the moisture got into the samples on Earth.

Now a remote sensing instrument on Chandrayaan-1, India's first mission to lunar orbit, has confirmed that the signal was real.

Two other spacecraft to look at the Moon - Nasa's Deep Impact probe and the US-European Cassini satellite - back up Chandrayaan.

Both collected their Moon data long before Chandrayaan was launched (in the case of Cassini, 10 years ago), but the significance of what they saw is only now being realised.

The quantity of water is seen to increase the closer the observations are made to the poles.

WATER ICE

Scientists suspect the water is created in the soil in an interaction with the solar wind, the fast-moving stream of particles that constantly billows away from the Sun.

Harsh space radiation triggers a chemical reaction in which oxygen atoms already in the soil acquire hydrogen nuclei to make water molecules and the simpler hydrogen-oxygen (OH) molecule.

The amounts are small, say researchers, but boost the notion that astronauts based on the Moon could use it as a resource.

"If it is a little or a lot, it's easy enough to split into hydrogen and oxygen and then you have rocket fuel," said Professor Taylor, a University of Tennessee researcher working on Chandrayaan.

The Indian Moon mission was launched late last year but has already stopped working due to a fault. Nevertheless, the Indian space agency (Isro) will consider the water discovery a major triumph and a vindication of its endeavours.

A Nasa probe is due to impact the Cabeus A crater near the Moon's south pole next month to see if it can kick up sufficient soil so that another satellite and Earth-based telescopes can detect the presence of water in the dusty plume.

Researchers say the latest water results give them confidence that the experiment performed by the Lunar Crater Observation and Sensing Satellite mission, known as LCROSS, could have a positive result.

They speculate that the water seen elsewhere on the lunar surface may migrate to the slightly cooler poles, much as water vapour on Earth will be drawn to a cold surface and condense.

This cold sink effect could result in vast quantities of water being retained in permanently shadowed craters in the form of ice, especially if it has being supplemented by water delivered comets.

Nasa's Lunar Prospector probe in the 1990s saw a strong hydrogen signal in the far north and south. Some scientists on the mission suggested there could be up to 300 million tonnes of water-ice buried in crater soils that never see sunlight.

Chandrayyan made its observations using a US-provided instrument, the Moon Mineralogy Mapper, or M3 for short.

The M3 assessed the nature of lunar soils by analysing the way that light from the sun was reflected off the surface.