US scientists have found evidence that water was held in the Moon's interior, challenging some elements of the theory of how Earth's satellite formed.
The Moon is thought to have been created in a violent collision between Earth and another planet-sized object.
Scientists thought the heat from this impact had vaporised all the water.
But a new study in Nature magazine shows water was delivered to the lunar surface from the interior in volcanic eruptions three billion years ago.
This suggests that water has been a part of the Moon since its early existence.
The discovery came from lunar volcanic glasses, pebble-like beads collected and returned to Earth by the US Apollo missions in the late 1960s and early 1970s.
In the decades since, scientists have sought to determine the nature of a class of chemical elements known as volatiles in the multicoloured glasses.
In particular, they searched the glasses for signs of water - but the evidence has remained elusive. This was consistent with a general consensus that the Moon was dry.
The team, from Brown University, the Carnegie Institution for Science, and Case Western Reserve University, used secondary ion mass spectrometry (SIMS) technology to detect extremely minute quantities of water in glasses and minerals.
"We developed a way to detect as little as five parts per million of water," said Erik Hauri, from the Carnegie Institution in Washington DC.
"We were really surprised to find a whole lot more in these tiny glass beads, up to 46 parts per million."
Lost to space
The team then confirmed through a series of tests that hydrogen had been present all along, and the samples had not been infused by hydrogen-rich solar winds or tainted by other volatiles.
"This confirms that water comes from deep within the mantle of the Moon," said lead author Alberto Saal, assistant professor of geological sciences at Brown University.
"It has nothing to do with secondary processes, such as contamination or solar wind."
The researchers believe the water was contained in magma which erupted via "fire fountains" on to the lunar surface more than three billion years ago.
About 95% of this water vapour was lost during the volcanic activity.
"Since the Moon was thought to be perfectly dehydrated, this is a giant leap from previous estimates," said co-author Erik Hauri, from the Carnegie Institution.
"It suggests the intriguing possibility that the Moon's interior might have had as much water as the Earth's upper mantle. But even more intriguing: If the Moon's volcanoes released 95% of their water, where did all that water go?"
Since the Moon's gravity is too feeble to retain an atmosphere, the researchers speculate that some of the water vapour from the eruptions was probably forced into space.
But some may also have drifted towards the cold poles of the Moon where ice may persist in permanently shadowed craters.
The research also might yield additional insight into how long water had been on Earth.
"It suggests that water was present within the Earth before the giant collision that formed the Moon," Dr Saal explained.
"That points to two possibilities: Water either was not completely vaporised in that collision or it was added a short time - less than 100 million years - afterward by volatiles introduced from the outside, such as with meteorites."
The glasses analysed in the study colour the lunar soil green or orange. While most glasses collected on the Moon were produced by melting of rocks during meteorite impacts, these lack the typical characteristics of so-called impact glasses.
In addition, their ages are similar to those of surrounding basaltic rocks, linking them to the Moon's geological evolution.
Verifying that water is at the Moon's poles is one goal of Nasa's Lunar Reconnaissance Orbiter mission, due to launch later this year.
It is the primary objective of the Lunar Crater Observation and Sensing Satellite (LCross) scheduled for launch in 2009.