Biogenic oxygen from Earth transported to the Moon
by a wind of magnetospheric ions
30 January 2017
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 1 , complicated oxygen isotope fractionation in lunar metal 2,3 (particularly the provenance of a 16O-poor component) remains an enigma 4,5 . 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 atmosphere 6 , 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 3,4 . We suggest the possibility that the Earth’s atmosphere of billions of years ago may be preserved on the present-day lunar surface.
In 2008, the Moon and the Japanese lunar orbiter Kaguya both lay within the Earth’s plasma sheet for tens of minutes to a few hours per month (Fig. 1, Table 1). When the Moon moved into the central magnetosphere on 21 April 2008, Kaguya measured plasma sheet ions with an energy of several kiloelectronvolts during the periods of 0:50–1:10 ut and 8:00–16:00 ut, and detected a weak signature of cold lobe ions in the remaining period (Fig. 2). Moonward-moving magnetospheric ions were observed by the Ion Energy Analyzer (IEA), whereas the Ion Mass Analyzer (IMA) measured both the ions coming from the Moon (anti-moonward ions) and the magnetospheric ions by means of a wide energy field of view (FOV) 7 .