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Breath of Life

19 December 2019

Conceptual image of Earth during its early bombardment stage.NASA’s Goddard Space Flight Center Conceptual Image Lab
Conceptual image of Earth during its early bombardment stage.

Among the planets, Earth is unique for its oxygen-rich atmosphere. None of the other terrestrial planets contain much oxygen in their atmosphere, despite oxygen being a rather common element in the cosmos. Part of the reason for this is that oxygen binds easily with other elements. With carbon to produce the carbon dioxide seen in the thick air of Venus, or with iron to form the rusty soil of Mars. So why is oxygen so freely available on Earth?

Early Earth didn’t have free oxygen in its atmosphere. The first life to appear on Earth was anaerobic, meaning that it didn’t require oxygen to survive. Then about 2.4 billion years ago life on Earth started liberating oxygen into the air. Much of it combined with iron on the surface to create a layer of rust we still see in the geological record.

But that was only the first oxygenation event, and it nearly killed off life on Earth. Two others occurred about 800 million years ago and 450 million years ago, and could have been caused by changes in tectonic activity.

This suggests that Earth’s free oxygen (and hence complex life) is due to an unusual combination of circumstances. If that’s the case, then Earth-like worlds could be exceptionally rare in the universe. This “rare-earth” idea could explain why we don’t see evidence of advanced alien civilizations.

But a new study in Science suggests that planets with rich oxygen atmospheres could be common.1 The team looked at models of marine biochemistry. They found that there is a feedback cycle in marine phosphorus. As early life liberates oxygen, the cycle allows for organisms to more effectively liberate oxygen. If that’s true, then the rise of oxygen on water-rich planets could be inevitable once life gains a foothold. Geologic events may shorten or lengthen the overall timeline a bit, but eventually, a planet will be capable of sustaining more complex life.

Life in the universe could still be uncommon if it is difficult for early life to appear on a planet. But once life does appear, it is always able to make its world home in time.


  1. Alcott, Lewis J., Benjamin JW Mills, and Simon W. Poulton. “Stepwise Earth oxygenation is an inherent property of global biogeochemical cycling.” Science 366.6471 (2019): 1333-1337. ↩︎