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

Abstract

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 composition1, complicated oxygen isotope fractionation in lunar metal2,3 (particularly the provenance of a 16O-poor component) re­mains an enigma4,5. 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 atmosphere6, 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 depth3,4. We suggest the possibility that the Earth’s atmosphere of billions of years ago may be preserved on the present-day lunar surface. The authors present evidence that the lunar soil contains oxygen ions from Earth that escape our atmosphere and reach the Moon through the Earth’s magnetospheric wind. The lunar surface possibly contains clues about the history of Earth’s atmosphere.