The case for methane as a biosignature stems from its instability in the planet’s atmosphere. Because photochemical reactions destroy atmospheric methane, it must be steadily replenished to maintain high levels.
“Oxygen is often talked about as one of the best biosignatures, but it’s probably going to be hard to detect with the NASA/ESA/CSA James Webb Space Telescope,” said Maggie Thompson, a graduate student at the University of California, Santa Cruz.
“Despite some prior studies on methane biosignatures, there had not been an up-to-date, dedicated assessment of the planetary conditions needed for methane to be a good biosignature.”
“We wanted to provide a framework for interpreting observations, so if we see a rocky planet with methane, we know what other observations are needed for it to be a persuasive biosignature.”
In their study, Thompson and colleagues examined a variety of non-biological sources of methane and assesses their potential to maintain a methane-rich atmosphere.
These include volcanoes; reactions in settings such as mid-ocean ridges, hydrothermal vents, and tectonic subduction zones; and comet or asteroid impacts.
“If you detect a lot of methane on a rocky planet, you typically need a massive source to explain that,” said Dr. Joshua Krissansen-Totton, also from the University of California, Santa Cruz.
“We know biological activity creates large amounts of methane on Earth, and probably did on the early Earth as well because making methane is a fairly easy thing to do metabolically.”
Nonbiological sources, however, would not be able to produce that much methane without also generating observable clues to its origins.
Outgassing from volcanoes, for example, would add both methane and carbon monoxide to the atmosphere, while biological activity tends to readily consume carbon monoxide.
The researchers found that nonbiological processes cannot easily produce habitable planet atmospheres rich in both methane and carbon dioxide and with little to no carbon monoxide.
They emphasized the need to consider the full planetary context in evaluating potential biosignatures.
They concluded that, for a rocky planet orbiting a sun-like star, atmospheric methane is more likely to be considered a strong indication of life if the atmosphere also has carbon dioxide, methane is more abundant than carbon monoxide, and extremely water-rich planetary compositions can be ruled out.
“One molecule is not going to give you the answer — you have to take into account the planet’s full context,” Thompson said.
“Methane is one piece of the puzzle, but to determine if there is life on a planet you have to consider its geochemistry, how it’s interacting with its star, and the many processes that can affect a planet’s atmosphere on geologic timescales.”
The scientists also considered a variety of possibilities for false positives and provides guidelines for assessing methane biosignatures.
“There are two things that could go wrong — you could misinterpret something as a biosignature and get a false positive, or you could overlook something that’s a real biosignature,” Dr. Krissansen-Totton said.
“With this paper, we wanted to develop a framework to help avoid both of those potential errors with methane.”
The team’s paper will be published in the Proceedings of the National Academy of Sciences.
Maggie Thompson et al. 2022. The case and context for atmospheric methane as an exoplanet biosignature. PNAS, in press; doi: 10.1073/pnas.2117933119
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