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Habitable worlds: New evidence for methane as an extraterrestrial sign of life

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A team led by UW postdoctoral student Nick Wogan has published a paper explaining why an abundance of methane in the atmosphere of an exoplanet (any planet orbiting a star other than the Sun) might be a potential indication of life. 

Scientists typically search for molecular oxygen as an indication of life (or of conditions favorable to life) on other planets, but unfortunately, the James Webb Space Telescope, set to launch October 2021, isn’t well equipped to detect it in the atmospheres of faraway planets. The new telescope, however, intended to replace the aging Hubble, is particularly adept at detecting atmospheric methane and carbon dioxide abundances. 

During the Archean, an eon early in Earth’s history, the first microbes developed and began to convert carbon monoxide into methane. This process continues today. As a result, methane began to build up in the atmosphere and has remained as an indication of biologic activity on Earth ever since.

However, life is not the only process we know of that can produce methane. Volcanism, deep sea hydrothermal vents, and meteor impacts can all generate methane as well. Wogan set out to determine whether volcanic gas emissions on terrestrial exoplanets were abundant enough to disguise any biologically produced methane. 

To do this, the group ran many combinations of simulations that modeled a wide range of volcanic chemistries possible for a terrestrial planet. 

“We wanted to understand whether if we look at another planet, if we see methane there, is that because of life, or is that because of some weird volcano that also produces methane?” Wogan said. 

Volcanic activity is only capable of producing less than 3% of the methane emissions found in today’s biosphere, supporting the claim that a proportionately large amount of methane in a planet’s atmosphere might be an indication that Earth-like organisms exist there, Wogan said. 

The researchers found that while deep sea vents and meteor impacts did produce methane, they weren’t capable of producing abundances near the level of biogenic methane.

In a few specific simulations, the researchers found that the volcanic methane flux actually did approach biologic levels, but such false positive cases can be determined from other contextual clues. If a volcano produces a lot of methane, chances are it will produce even more carbon monoxide, which would be a detectable indication of increased volcanic methane production.

Detecting large amounts of methane by itself isn’t enough evidence to support claims of life. Large amounts of carbon dioxide alongside the methane would strengthen the possibility of a biosignature, as chemically Earth-like planets commonly produce oxygen-based gases. Therefore, we would expect to find abundant oxygen-rich gases, such as carbon dioxide and water vapor, in the atmosphere of a life-supporting exoplanet.

“Really, our best shot of finding evidence of life on another planet is probably seeing the combination of methane and carbon dioxide,” Wogan said.

Looking for further evidence of biogenic methane, the researchers would expect that an inhabited planet would have low levels of atmospheric carbon monoxide. This is because Archean methane-producing bacteria or other similar lifeforms would likely consume the carbon monoxide and keep it from building up in the atmosphere. 

The group’s findings will be particularly helpful to astronomers analyzing exoplanetary atmospheres with the James Webb Space Telescope, and may be instrumental in finding extraterrestrial methane biosignatures.

Reach reporter Sarah Kahle at news@dailyuw.com. Twitter: @karahsahle

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(1) comment

mollykstark

very cool sarah

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