Biogenic oxygen from Earth transported to the Moon by a wind of magnetospheric ions

For five days of each lunar orbit, the Moon is shielded from solar wind bombardment by the Earths magnetosphere, which is filled with terrestrial ions. Although the possibility of the presence of terrestrial nitrogen and noble gases in lunar soil has been discussed based on their isotopic composition, complicated oxygen isotope fractionation in lunar metal(2,3) (particularly the provenance of a O-16-poor component) remains an enigma(4,5) . Here, we report observations from the Japanese spacecraft Kaguya of significant numbers of 110 keV O+ ions, seen only when the Moon was in the Earths plasma sheet. Considering the penetration depth into metal of O+ ions with such energy, and the O-16-poor mass-independent fractionation of the Earths upper atmosphere 6 , we conclude that biogenic terrestrial oxygen has been transported to the Moon by the Earth wind (at least 2.6 x104 ions cm(-2) s(-1)) and implanted into the surface of the lunar regolith, at around tens of nanometres in depth (3,4) . We suggest the possibility that the Earths atmosphere of billions of years ago may be preserved on the present-day lunar surface.