Methane could be the most reliable sign of alien life, study says

Methane is best known as a powerful greenhouse gas, but could be a more reliable sign of life beyond Earth than oxygen, a new study claims. 

According to researchers in California, the detection of methane in another planet’s atmosphere could be considered a more ‘compelling sign of life’.  

Methane (CH4) is one of the few potential signs of life, or ‘biosignatures’, that could be readily detectable with the James Webb Space Telescope (JWST). 

JWST could detect different infrared wavelengths that indicate chemicals such as water and methane present in the atmosphere of ‘exoplanets’ – planets outside our solar system.  

JWST could help reveal whether methane is coming from a biological source (a living organism) or a non-biological source (such as a volcano) on an exoplanet. 

Methane in a planet’s atmosphere may be a sign of life if nonbiological sources can be ruled out. This illustration summarizes the known abiotic sources of methane on Earth, including outgassing from volcanoes, reactions in settings such as mid-ocean ridges, hydrothermal vents, and subduction zones, and impacts from asteroids and comets.

The new study was led by researchers at University of California, Santa Cruz, with support from NASA. 

‘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 (UCSC) and lead author of the new study. 

Methane absorbs most strongly at near infrared red wavelengths (1-3 um), where JWST is most sensitive.  

On our planet, material in Earth’s mantle (its layer bounded below by a core and above by a crust) bonds with atmospheric oxygen, pulling it out of the atmosphere. 

If other rocky planets exist with similar processes, they may therefore not have much detectable oxygen in their atmosphere but still host some kind of biological life.  

This is one reason that planetary scientists instead focus on methane when searching for life on other planets.  

Methane, a powerful greenhouse gas, is known for its presence in burps and farts, especially from cattle. 

On Earth, methane is also produced by the bacterial decomposition of vegetable matter under water volcanoes and vents in the ocean floor.   

The researchers refer to methane as a ‘biosignature’ – an element that is indicative of past or present life. Methane is potentially the only biosignature gas detectable in the atmospheres of rocky planets with JWST, they say. 

Although non-biological processes can generate methane too, it’s thought that biological activity could be the source of methane in a rocky planet’s atmosphere, if certain chemical and biological boxes are ticked. 

Despite some prior studies on methane biosignatures, there had not been an up-to-date assessment of planetary conditions needed for methane to be a 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,’ Thompson said. 

For the study, the team examined a variety of non-biological sources of methane that are seen on Earth – volcanoes, reactions in mid-ocean ridges, hydrothermal vents, tectonic subduction zones and comet or asteroid impacts.  

On Earth, human activities emitting methane include the raising of livestock and natural wetlands (file photo)

For each methane source, the team assessed their potential to maintain a methane-rich atmosphere on a rocky planet.   

Because photochemical reactions (those initiated by the absorption of energy from light) 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 study co-author Joshua Krissansen-Totton at UCSC. 

‘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.’  

Non-biological sources, however, would not be able to produce that much methane without also revealing the presence of other gases, and in turn their identity as non-biological sources.  

For example, outgassing from a volcano (a non-biological source) would add both methane and carbon monoxide to the atmosphere; conversely, biological activity tends to readily consume carbon monoxide. 

Overall, the researchers found that non-biological processes cannot easily produce habitable planet atmospheres that are rich in both methane and carbon dioxide (CO2) and that have little to no carbon monoxide. 

For that reasons, atmospheric methane is more likely to be considered a strong indication of life on a rocky planet if the atmosphere also has CO2, and if methane is more abundant than carbon monoxide.  

In particular set of circumstances, a persuasive case could be made for biological activity as the source of methane in a rocky planet’s atmosphere. Pictured is an artist’s impression of what a rocky planet, similar to Earth, may look like

‘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,’ Thompson said. 

The study also considers a variety of possibilities for ‘false positives’ – results that could incorrectly indicate alien life – 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,’ Krissansen-Totton said.

‘With this paper, we wanted to develop a framework to help avoid both of those potential errors with methane.’ 

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 said.

‘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 nonbiological methane production.’ 

The study has been published today in Proceedings of the National Academy of Sciences.