Water particles have been detected on the surface of the Moon by three missions, including an Indian probe.
The evidence, disclosed in new scientific papers, overturns the long accepted view that lunar soil is dry and comes just two weeks before a NASA probe is to crash into the surface near the Moon's southern pole to see if water can be detected in the dust and debris released by the impact.
The new data was gathered by probes equipped with NASA instruments designed to map the Moon's mineral composition.
The so-called "Moon Mineralogy Mapper," or M3, uses the reflection of sunlight off the Moon's surface to determine soil composition.
In one of the three papers published in the latest edition of the journal Science, researchers said they analyzed light waves detected by an M3 instrument on board an Indian satellite, Chandrayyan-1.
The reflected light waves indicated a chemical bond between oxygen and hydrogen -- proof, the researchers said, of the existence of water on the Moon's surface.
Larry Taylor of the University of Tennessee, one of the study's co-authors, said the instrument is capable of detecting the composition of the thin upper layer of the Moon's surface only to a depth of two or three inches.
Until now, scientists had advanced the theory that there might be ice at the permanently dark bottom of craters at the Moon's poles but that the rest of the Moon was totally dry.
Lunar rocks and soil contain about 45 percent oxygen, but the source of the hydrogen observed by the instruments on the three probes remains to be determined.
Taylor and his colleagues believe it may have come from an astronomical phenomenon called the solar wind, which consist mainly of streams of positively charged hydrogen atoms emitted as the sun undergoes nuclear fusion.
They estimate that each ton of lunar soil consists of 25 percent water.
Two other probes equipped with M3-type instruments also detected the chemical signature for the presence of water.
These include data gathered by the American spacecraft Cassini as it passed near the moon a decade agon on its way to Saturn.
The third probe, also American, was Deep Impact, which was launched toward the comet Tempel-1 in 2005 to pierce it with a projectile in order to analyze the dust cloud created by the impact.
Deep Impact passed near the Moon to gather data with an instrument similar to M3.
First drops in 1960
Samples of lunar rock and soil brought back to Earth by Apollo astronauts in the 1960s also contained traces of water.
But the containers in which they were transported were not hermetically sealed so researchers dismissed the presence of water as coming from the Earth.
"To some extent, we were fooled," said Taylor, who has studied the original Apollo missions. "Since the boxes leaked, we just assumed the water we found was from contamination with terrestrial air."
Indian scientists lost radio contact with the Chandrayaan-1 lunar satellite last month, but it had already collected enough data to provide the firmest evidence so far of water concentrated near the lunar poles.
"To find water on the moon was one of the main objectives," mission director Mylswamy Annadurai told AFP in Bangalore.
"The baby has done its job," a clearly delighted Annadurai said. "It's a major milestone, although we still have to quantify the findings."