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.
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We thank all the members of the SELENE project. This work was partly supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grants (No. 26800258, 26610183 and 22224010), the Mitsubishi Foundation and Yamada Science Foundation. Our discussion was partially supported by the project entitled “On-Site Mass Spectrometry Based on the Multi-Turn Time-of-Flight Mass Spectrometer ‘MULTUM’ (PI: Michisato Toyoda)” in a Support Program for Osaka University Future Research Initiative.
The authors declare no competing financial interests.
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Terada, K., Yokota, S., Saito, Y. et al. Biogenic oxygen from Earth transported to the Moon by a wind of magnetospheric ions. Nat Astron 1, 0026 (2017). https://doi.org/10.1038/s41550-016-0026
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