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.