Water on the Moon came in large part from comets which bombarded the lunar surface in its infancy, a new study suggests.
For decades the Moon, as well as being devoid of life and lacking an atmosphere, was thought to have been completely waterless.
NASA findings last year of significant traces of frozen water in a permanently shadowed crater have extinguished that assumption.
In a paper published in the journal Nature Geoscience, astrophysicists led by James Greenwood of Wesleyan University in Connecticut analysed rock samples collected during the Apollo expeditions. They were especially interested in variations in hydrogen isotopes in a water-loving mineral called apatite.
The signature, they say, points to three potential sources: from the sub-surface lunar mantle, from protons brought by the "solar wind" of particles blasted from the Sun - and from comets.
The isotope measurements in the apatite were similar to those previously found in three well-known comets: Hale-Bopp, Hyakutake and Halley.
Comets have been described as frozen water reservoirs orbiting the Sun, because a comet's nucleus contains vast quantities of ice.
Under a "giant impact" theory dating back to the 1970s, the Moon was formed from part of Earth, after our planet collided with a space rock or planet some 4.5 billion years ago.
"Significant delivery" of cometary water occurred after the Moon-forming event, Greenwood's team suggests.
Comets also provided Earth with some of its water as well as key chemicals to kickstart life, according to some hypotheses.