For five days every month, the Earth's magnetic field shields the Moon from the solar wind. During this time, oxygen ions from the Earth's atmosphere can reach the Moon and bury themselves in the soil. Consequently, a paper in Nature Astronomy proposes that the Moon conserves a record of the air that surrounded the Earth at different times – one that may help us reconstruct the development of life on Earth.

In 2008, the Kaguya lunar orbiter collected samples of particles. While inside the Earth's plasma sheet, a tail of plasma opposite the Sun from the Earth, these particles included oxygen and hydrogen atoms from the Earth's atmosphere that had lost an electron, making them positively charged ions. When either Kaguya or the Moon itself was outside the Earth's magnetosphere, these were not detected as the solar wind dominated.

First author Professor Kentaro Terada of Osaka University and colleagues provide evidence that the higher energy O+ ions Kaguya detected do not come from the solar wind and therefore must have come from the Earth's ionosphere, the outermost part of the atmosphere where radiation knocks electrons off atoms.

These ions have enough energy that they penetrate solid material, including the regolith that covers the surface of the Moon. In the more energetic cases, they should reach depths of 30-40 nanometers.

These would be indistinguishable from other lunar oxygen, were it not that the process of ozone formation in the stratosphere results in higher concentrations of heavier oxygen isotopes (¹⁷O and ¹⁸O) than exists elsewhere in the Solar System. Consequently, Terado argues, the oxygen ions burying themselves in the lunar surface are skewed towards these isotopes, at the expense of the more common Oxygen-16.

This resolves a longstanding mystery. Some of the lunar material brought back by the Apollo missions has a surprising amount of ¹⁷O and ¹⁸O, something that has been unexplained until now. Even when mixed with oxygen from the Moon's formation, some deposited by meteorites and a component from the solar wind, Terada argues that the component from the Earth's atmosphere has left a detectable trace.

The significance of this discovery lies in the fact that the Earth's atmosphere has not always been rich in oxygen. It took the rise of photosynthetic microbes to create an oxygen-rich atmosphere, so life's imprint reached the Moon billions of years before we did.

There is still debate as to when the atmosphere oxygenated, with available evidence producing conflicting dates. Meanwhile, the Moon may have been acting as a storehouse, just waiting for us to get there to examine it. These records, if we can measure them finely enough, may help us date important developments in the history of life more accurately.