Earth had almost no atmosphere when it formed 4.6 billion years ago from a hot mix of gases and solids. The ground was molten. As the Earth cooled, an atmosphere formed, primarily from gases emitted by volcanoes. It contained hydrogen sulfide, methane, and ten to 200 times the amount of carbon dioxide found in today’s atmosphere. After approximately half a billion years, the Earth’s surface cooled and solidified sufficiently for water to collect on it. A new study examines the planetary context in which the detection of methane in the atmosphere of an exoplanet could be regarded as a compelling sign of life.
If life is abundant in the universe, atmospheric methane could be the first sign of life detected by astronomers beyond Earth. Although nonbiological processes can produce methane, a new study by UC Santa Cruz scientists establishes a set of circumstances under which a compelling case can be made for biological activity as the source of methane in the atmosphere of a rocky planet.
This is significant because methane is one of the few potential signs of life, or “biosignatures,” that may be detectable with the James Webb Space Telescope, which will begin observations later this year.
“Oxygen is often talked about as one of the best biosignatures, but it’s probably going to be hard to detect with JWST,” said Maggie Thompson, a graduate student in astronomy and astrophysics at UC Santa Cruz and lead author of the new study.
This study is focused on the most obvious false positives for methane as a biosignature. The atmospheres of rocky exoplanets are likely to surprise us, and we will need to be cautious in our interpretations. Future research should attempt to anticipate and quantify more unusual mechanisms for nonbiological methane production.
Joshua Krissansen-Totton
Despite previous research on methane biosignatures, there had not been a comprehensive assessment of the planetary conditions required for methane to be a good biosignature. “We wanted to provide a framework for interpreting observations,” Thompson explained. “If we see a rocky planet with methane, we know what other observations are required for it to be a convincing biosignature.”
The study, which was published in the Proceedings of the National Academy of Sciences, examines a variety of non-biological methane sources and assesses their ability to maintain a methane-rich atmosphere. Volcanoes, reactions in mid-ocean ridges, hydrothermal vents, and tectonic subduction zones, and comet or asteroid impacts are examples of these.
The case for methane as a biosignature stems from its instability in the atmosphere. Because photochemical reactions destroy atmospheric methane, it must be steadily replenished to maintain high levels.
“If you detect a lot of methane on a rocky planet, you typically need a massive source to explain that,” said coauthor and UCSC Sagan Fellow Joshua Krissansen-Totton. “We know that biological activity produces large amounts of methane on Earth, and that it probably did on the early Earth as well, because producing methane is a relatively simple metabolic process.”
Nonbiological sources, on the other hand, could not produce that much methane without leaving observable traces of its origins. Volcanoes, for example, would emit both methane and carbon monoxide into the atmosphere, whereas biological activity readily consumes 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.
The study emphasizes the importance of considering the entire planetary context when assessing potential biosignatures. The researchers concluded that atmospheric methane is more likely to be considered a strong indication of life for a rocky planet orbiting a sun-like star if the atmosphere also contains carbon dioxide, methane is more abundant than carbon monoxide, and extremely water-rich planetary compositions can be ruled out.
“One molecule will not give you the answer; you must consider the entire context of the planet,” Thompson explained. “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 study considers 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 is a true biosignature,” Krissansen-Totton explained. “With this paper, we wanted to develop a framework to help avoid both of those potential methane errors.”
He added that there is still a lot of work to be done to fully understand any future methane detections. “This study is focused on the most obvious false positives for methane as a biosignature,” he explained. “The atmospheres of rocky exoplanets are likely to surprise us, and we will need to be cautious in our interpretations. Future research should attempt to anticipate and quantify more unusual mechanisms for nonbiological methane production.”
