A team of astrobiologists from the Universities of Washington and California has found a simple approach to look for alien life that might be more promising than just looking for oxygen on extrasolar planets.
Future telescopes like the NASA/ESA/CSA James Webb Space Telescope (right) will observe the atmospheres of distant planets to seek evidence of life. Earth (top left) has several gases in its atmosphere that reveal the presence of life, primarily oxygen and ozone. Krissansen-Totton et al find that for the early Earth (bottom left), the combination of abundant methane and carbon dioxide would provide an alternative sign of life. Image credit: NASA / Wikimedia Commons / Joshua Krissansen-Totton.
“The idea of looking for atmospheric oxygen as a biosignature has been around for a long time,” said Joshua Krissansen-Totton, a doctoral student at the University of Washington.
“And it’s a good strategy — it’s very hard to make much oxygen without life.”
“But we don’t want to put all our eggs in one basket. Even if life is common in the cosmos, we have no idea if it will be life that makes oxygen. The biochemistry of oxygen production is very complex and could be quite rare.”
Krissansen-Totton and co-authors looked at the history of life on Earth to find times where the planet’s atmosphere contained a mixture of gases that are out of equilibrium and could exist only in the presence of living organisms — anything from pond scum to giant redwoods.
In fact, life’s ability to make large amounts of oxygen has only occurred in the past one-eighth of Earth’s history.
By taking a longer view, the team identified a new combination of gases that would provide evidence of life: methane plus carbon dioxide, minus carbon monoxide.
“We need to look for fairly abundant methane and carbon dioxide on a world that has liquid water at its surface, and find an absence of carbon monoxide,” said Professor David Catling, also from the University of Washington.
“Our study shows that this combination would be a compelling sign of life. What’s exciting is that our suggestion is doable, and may lead to the historic discovery of an extraterrestrial biosphere in the not-too-distant future.”
The researchers looked at all the ways that a planet could produce methane — from asteroid impacts, outgassing from the planet’s interior, reactions of rocks and water — and found that it would be hard to produce a lot of methane on a rocky, Earth-like planet without any living organisms.
If methane and carbon dioxide are detected together, especially without carbon monoxide, that’s a chemical imbalance that signals life. The carbon atoms in the two molecules represent opposite levels of oxidation. Carbon dioxide holds as many oxygen molecules as it can, while the carbon in methane lacks oxygen and instead has oxygen’s chemical adversary, hydrogen.
“So you’ve got these extreme levels of oxidation. And it’s hard to do that through non-biological processes without also producing carbon monoxide, which is intermediate,” Krissansen-Totton said.
“For example, planets with volcanoes that belch out carbon dioxide and methane will also tend to belch out carbon monoxide.”
“What’s more, carbon monoxide tends not to build up in the atmosphere of a planet that harbors life.”
“Carbon monoxide is a gas that would be readily eaten by microbes. So if carbon monoxide were abundant, that would be a clue that perhaps you’re looking at a planet that doesn’t have biology.”
The authors agree that oxygen is a good way to look for signs of life, but think that this new combination is at least as likely to pop up through the new telescopes’ sights.
“Life that makes methane uses a simple metabolism, is ubiquitous, and has been around through much of Earth’s history,” Krissansen-Totton said.
“It’s an easy thing to do so it’s potentially more common than oxygen-producing life. This is definitely something we should be looking for as new telescopes come online.”
The team’s paper appears in the January 24, 2018 issue of the journal Science Advances.
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Joshua Krissansen-Totton et al. 2018. Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life. Science Advances 4 (1): eaao5747; doi: 10.1126/sciadv.aao5747
