Life on Earth may have made its mark on the moon billions of years before Neil Armstrong’s famous first step.

Observations by Japan’s moon-orbiting Kaguya spacecraft suggest that oxygen atoms from Earth’s upper atmosphere bombard the moon’s surface for a few days each month. This oxygen onslaught began in earnest around 2.4 billion years ago when photosynthetic microbes first flourished (SN Online: 9/8/15), planetary scientist Kentaro Terada of Osaka University in Japan and colleagues propose January 30 in Nature Astronomy.

The oxygen atoms begin their incredible journey in the upper atmosphere, where they are ionized by ultraviolet radiation, the researchers suggest. Electric fields or plasma waves accelerate the oxygen ions into the magnetic cocoon that envelops Earth. One side of that magnetosphere stretches away from the sun like a flag in the wind. For five days each lunar cycle, the moon passes through the magnetosphere and is barraged by earthly ions, including oxygen.

Based on Kaguya’s measurements of this space-traveling oxygen in 2008, Terada and colleagues estimate that at least 26,000 oxygen ions per second hit each square centimeter of the lunar surface during the five-day period. The uppermost lunar soil may, therefore, preserve bits of Earth’s ancient atmosphere, the researchers write, though determining which atoms blew over from Earth or the sun would be difficult.

BY THOMAS SUMNER

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