Research press release


Nature Astronomy

Footprints of life on the Moon



今回、大阪大学の寺田 健太郎(てらだ けんたろう)の研究チームは、月と月周回衛星「かぐや」の両方が太陽風から遮られるときに衛星から得たデータの解析によってこの謎を解いた。寺田たちは「地球風」が酸素イオンを運んで月にまで届き、月表土の深さ1マイクロメートルという浅い層に入り込んでいるという明らかな証拠を発見した。また、地球の大気中に酸素がかなりの存在度で出現して以来、およそ25億年前からこの過程が起こり続けていることも確認された。


The Moon has been periodically showered with oxygen from Earth for billions of years, reports a study published online in Nature Astronomy this week. The study suggests that lunar soil could potentially preserve a historical record of the Earth’s atmosphere.

The solar wind of energetic particles hits the surface of the Moon continuously, except for five days each lunar orbit, when the Earth’s magnetic field stands between the Sun and the Moon and deflects these particles. During this period, ions from Earth can reach the Moon. Although this process had been proposed for nitrogen and noble gases (based on the isotopic composition of lunar soils), there has not yet been definitive proof for Earth’s most important gas: oxygen.

Kentaro Terada and colleagues solve this enigma by analysing data from the Kaguya lunar orbiter, obtained when both the Moon and the spacecraft were shielded by the solar wind. They found clear evidence that ‘Earth wind’ is reaching the Moon carrying oxygen ions, which then get stuck in a micrometre-deep shallow layer of lunar soil. They identified that this process has been happening since oxygen first appeared in significant abundances in Earth’s atmosphere, approximately 2.5 billion years ago.

As most oxygen on Earth is generated by the biosphere, this result suggests that the Moon has been continuously ‘contaminated’ by life-generated products for a good part of its history. It also means that the ancient atmosphere of the Earth might be preserved in lunar soil, although the authors stress that it would be difficult to disentangle the contributions of the solar wind and the Earth wind.

doi: 10.1038/s41550-016-0026 | 英語の原文

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