Their discovery suggests that water was present deep within the moon when the pebbles were formed during violent lunar eruptions 3.3 to 3.6 billion years ago. ''Most people believed that the moon was dry,'' said Alberto Saal at Brown University in Rhode Island, who was part of the team that did the research. ''People had tried to measure for 40 years and couldn't see any evidence for water ... they were not convinced we were doing something worthwhile.'' The analysis, published today in the journal Nature, was a surprise because scientists believe that the moon formed when a Mars-sized planet collided with the infant Earth. Saal and his colleagues used the extremely sensitive technique to analyse rocks collected during the Apollo 15 mission in 1971 and the last visit by people to the moon, Apollo 17 in 1972.
Testing the interior of the pebbles to avoid surface contamination, scientists found trace amounts of hydrogen along with chlorine and fluorine, which, like carbon and sulfur, are found in rocks from explosive eruptions.
The discovery of hydrogen alongside other elements suggested to scientists that water came from inside the moon and not from an external source, such as a comet.
Based on the amount of hydrogen found in the pebbles, scientists estimated the lunar magma contained 260 to 745 parts per million of water, similar to what is found in the Earth's upper mantle.
Finding water is crucial to NASA's plans for moon exploration.
''This could be really important if you want to put a base (on the moon),'' said Saal. The extracted water could be used for thirsty astronauts and to create hydrogen to use as fuel.
The finding throws at least a little water on the currently favoured hypothesis concerning the moon's origin. Many scientists think that the moon was formed when a large proto-planet slammed into Earth, sending into space molten debris that eventually became the moon.
Scientists have long assumed that the heat created by the collision would have vaporised any water present and that the small gravitational field of the primeval moon would not have been strong enough to recapture the vapoar.