Why comets have oxygen atmospheres

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Comet 67P/Churyumov–Gerasimenko, as seen by Rosetta.

Why would a comet have oxygen in its atmosphere? That’s the question that has been troubling scientists since the European Space Agency’s Rosetta spacecraft visited the comet 67P/Churyumov-Gerasimenko in 2015, and made the surprising discovery that the comet’s wispy-thin atmosphere did indeed contain molecular oxygen.

Molecular oxygen, otherwise known as the common gas O2 on Earth, where it is a byproduct of life, is rare in space: oxygen atoms are more likely to bond with hydrogen (to form water, H2O) or carbon (to form carbon dioxide, CO2).

The presence of O2 on the comet has been something of a mystery, but an answer has come from a surprising source: Konstantinos P. Giapis, a professor of chemical engineering at Caltech, whose work on collisions of high-speed ions with semiconductor surfaces has in the past been applied mainly in computer chip manufacture. “What I’ve been studying for years is happening right here on this comet,” Giapis says. 

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A visualisation of high-speed water molecules hitting oxidized silicon and iron surfaces and producing of a plume that includes molecular oxygen.

Giapis and Yunxi Yao, his co-author on a paper published in Nature Communications, modelled the conditions on the comet in their lab. They demonstrated that, if water molecules flew off the comet under heating by the sun, the molecules could be ionised (split into their component atoms) by the solar radiation, which would also push them back toward the comet’s surface. At the surface, the loose oxygen atoms could meet and bond with oxygen atoms contained in materials such as rust and sand, forming O2.

This finding may affect research on planets beyond our solar system. Previously, the presence of molecular oxygen has been taken to be a good indicator of the likely presence of life, but that view may now change.

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