SANTA CRUZ, Calif. — Methane holds the key to finding alien life, according to new research. The powerful greenhouse gas is emitted by microbes. Most astronomers advocate looking for water and oxygen, but the recently launched James Webb Space Telescope (JWST) can detect methane and is due to begin observations in the summer.
The gas has been found wafting on Saturn’s moons Titan and Enceladus.
“If you detect a lot of methane on a rocky planet, you typically need a massive source to explain that,” says study co-author Dr. Joshua Krissansen-Totton, of the University of California, Santa Cruz, in a statement. “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.”
It can be the source of methane in a rocky planet’s atmosphere, the study shows. The research team assessed the planetary conditions needed for methane to be a good biosignature.
“Oxygen is often talked about as one of the best biosignatures, but it’s probably going to be hard to detect with JWST,” says lead author Maggie Thompson, a graduate student in astronomy and astrophysics at the university, in a statement. “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.”
What makes methane a possible biosignature?
Non-biological sources include volcanoes, ocean ridges, hydrothermal vents, tectonic subduction zones and comet or asteroid impacts. The case for methane as a biosignature stems from its instability in the atmosphere.
Photochemical reactions destroy atmospheric methane. It must be steadily replenished to maintain high levels. Nonbiological sources would not be able to produce that much methane without also generating observable clues to its origins. They cannot easily produce habitable planet atmospheres rich in both methane and carbon dioxide and with little to no carbon monoxide. Outgassing from volcanoes, for example, would add both methane and carbon monoxide to the atmosphere.
Biological activity tends to readily consume carbon monoxide. The study emphasises the need to consider the full planetary context. For a rocky planet orbiting a sun-like star, methane is more likely to indicate life if the atmosphere also has carbon dioxide.
“One molecule is not going to give you the answer – you have to take into account the planet’s full context,” says Thompson. “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.”
Beware of false positives
The study considers a variety of possibilities for “false positive”‘ 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,” notes Krissansen-Totton. “With this paper, we wanted to develop a framework to help avoid both of those potential errors with methane.”
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 adds. “The atmospheres of rocky exoplanets are probably going to surprise us, and we will need to be cautious in our interpretations. Future work should try to anticipate and quantify more unusual mechanisms for non-biological methane production.”
Titan has methane in its atmosphere and Enceladus has a liquid ocean with erupting plumes of gas and water.
The study is published in the journal Proceedings of the National Academy of Sciences.
Report by Mark Waghorn, South West News Service