By Jonathan Amos
Science reporter, BBC News
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 H2O 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 moon 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 water in the samples got in only after they were hauled back to Earth.
The only safe scientific conclusion they could draw at the time was that the lunar surface was all but bone dry.
Now a remote sensing instrument on Chandrayaan-1, India's first mission to lunar orbit, has confirmed that there is a real H2O signal at the Moon.
Two other satellites to look at the Moon - the US Deep Impact probe and the US-European Cassini spacecraft - back up Chandrayaan.
Both collected Moon data before Chandrayaan was launched (in the case of Cassini, 10 years ago), but the significance of what they saw is only now being fully realised.
The quantity of water is seen to increase the closer the observations are made to the poles - the very places the Apollo missions never went.
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.
The LCROSS experiment will target a crater near the south pole
"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, Knoxville, researcher who has worked on the Chandrayaan data.
The Indian Moon mission was launched late last year but radio communication with it was abruptly lost in August. Nevertheless, the Indian space agency (Isro) will consider the water discovery a major triumph and a vindication of its endeavours.
A US space agency (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 vapour in the dusty plume.
Researchers say the latest results, published online by the journal Science, give them confidence that the experiment performed by the Lunar Crater Observation and Sensing Satellite mission, known as LCROSS, could have a positive outcome.
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 condense on a cold surface.
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 by comets.
Nasa's Lunar Prospector probe in the late 1990s saw a strong hydrogen signal at high north and south latitudes. 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.
Chandrayaan 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.
It could only see the top few centimetres of soil. Nasa's Lunar Reconnaissance Orbiter, which is currently circling the Moon, has the capability to see down to nearly a metre. Its data could determine if the presence of water is much more extensive.
Dr Jim Garvin is the chief scientist at the US space agency's Goddard Space Flight Center.
He was asked if he thought the Moon had become an exciting place again for science.
"I think it always was; it's just we saw this big exciting Solar System and after touching the Moon with six human missions, we moved on - to Mars, to the outer planets, to comets and asteroids.
"And now we are rediscovering the enigmas of the Moon and they're really in our own backyard. They're tantalisingly close," he told BBC News.