Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon

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Water plays a critical role in the evolution of planetary bodies, and determination of the amount and sources of lunar water has profound implications for our understanding of the history of the Earth-Moon system. During the Apollo programme, the lunar samples were found to be devoid of indigenous water. The severe depletion of lunar volatiles, including water, has long been seen as strong support for the giant-impact origin of the Moon. Recent studies have found water in lunar volcanic glasses and in lunar apatite, but the sources of lunar water have not been determined. Here we report ion microprobe measurements of water and hydrogen isotopes in the hydrous mineral apatite, found in crystalline lunar mare basalts and highlands rocks collected during the Apollo missions. We find significant water in apatite from both mare and highlands rocks, indicating a role for water during all phases of the Moon's magmatic history. Variations of hydrogen isotope ratios in apatite suggest the lunar mantle, solar wind protons, and comets as possible sources for water in lunar rocks and imply a significant delivery of cometary water to the Earth-Moon system shortly after the Moon-forming impact.