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Physical Chemistry

Atmospheric imbalance may be best clue of extraterrestrial life

New telescopes might search for methane and carbon dioxide, not oxygen

by Sam Lemonick
January 29, 2018 | A version of this story appeared in Volume 96, Issue 5

Earth's atmosphere seen from space.
Credit: NASA
Life on Earth is responsible for chemical disequilibria in the atmosphere.

When the James Webb Space Telescope launches next year, it will look for evidence of life in the atmospheres of planets in distant solar systems. New research argues that looking for methane and carbon dioxide may be a better bet than looking for oxygen (Sci. Adv. 2018, DOI: 10.1126/sciadv.aao5747). Rather than look for a single molecule associated with life as we know it, like oxygen, Joshua Krissansen-Totton of the University of Washington and colleagues propose looking for evidence of chemical disequilibria. On Earth, methane and oxygen represent such a disequilibrium; without active production from living organisms such as plants and microbes, those two would not coexist in our atmosphere. Given enough time, they would react to form carbon dioxide and water. In the paper, Krissansen-Totton and colleagues reconstructed the chemical disequilibria that were present in Earth’s ocean and atmosphere starting about 4 billion years ago and during a more recent era with very different chemistry. They conclude that searching for evidence of methane and carbon dioxide as signs of these disequilibria may give us the best chance of finding extraterrestrial life. The problem with looking for oxygen in exoplanets’ atmospheres, according to Krissansen-Totton, is that it may not be very common. The biochemical processes that make oxygen are complex, and on Earth they have evolved only one time. And even if oxygen-producing organisms do exist, the molecule is so reactive that it can take a long time for a significant amount of oxygen to accumulate in a planet’s atmosphere. “On the other hand,” says Krissansen-Totton, “making methane is relatively easy in terms of biochemistry.” And, he adds, there’s evidence that organisms evolved that ability early in Earth’s history. That means astronomers may be more likely to spot it. As luck would have it, the James Webb telescope is better equipped to observe carbon dioxide and methane than oxygen.

CORRECTION: The story was updated Feb. 21, 2018, to correct the atmospheric disequilibrium seen on Earth. It involves methane and oxygen, not methane and carbon dioxide.



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