For five days of each lunar orbit, the Moon is shielded from solar wind bombardment by the Earth’s 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 (particularly the provenance of a ¹⁶O-poor component) re­mains an enigma. Here, we report observations from the Japanese spacecraft Kaguya of significant numbers of 1–10 keV O+ ions, seen only when the Moon was in the Earth’s plasma sheet. Considering the penetration depth into metal of O+ ions with such energy, and the ¹⁶O-poor mass-independent fractionation of the Earth’s upper atmosphere, we conclude that biogenic terrestrial oxygen has been transported to the Moon by the Earth wind (at least 2.6 × 10⁴ ions cm⁻² s−1) and implanted into the surface of the lunar regolith, at around tens of nanometres in depth. We suggest the possibility that the Earth’s atmosphere of billions of years ago may be preserved on the present-day lunar surface.

Photo by courtesy of Osaka University/NASA

(Link) http://resou.osaka-u.ac.jp/en/research/2017/20170131_1