NASA’s Cassini spacecraft has detected an unusually high concentration of methane, along with carbon dioxide and dihydrogen, in the moons of Saturn by flying through their plumes. The spacecraft has found that Titan has methane in its atmosphere and Enceladus has a liquid ocean with erupting plumes of gas and water.
An international research team has used new statistical methods to understand if methanogenesis or methane production by microbes could explain the molecular hydrogen and methane. The models combined geochemistry and microbial ecology to decode what possible processes could explain these observations.
A paper published last month in Nature Astronomy concluded that there may be unknown methane-producing processes on Enceladus that await discovery.
Are there methane-producing organisms on Earth?
Most of the methane on Earth has a biological origin. Microorganisms called methanogens are capable of generating methane as a metabolic byproduct. They do not require oxygen to live and are widely distributed in nature. They are found in swamps, dead organic matter, and even in the human gut. They are known to survive in high temperatures and simulation studies have shown that they can live in Martian conditions. Methanogens have been widely studied to understand if they can be a contributor to global warming.
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Could there be methanogens on Enceladus?
“Obviously, we are not concluding that life exists in Enceladus’ ocean,” said Régis Ferrière, one of the lead authors, in a release. “Rather, we wanted to understand how likely it would be that Enceladus’ hydrothermal vents could be habitable to Earth-like microorganisms. Very likely, the Cassini data tell us, according to our models.
And biological methanogenesis appears to be compatible with the data. He adds: “In other words, we can’t discard the ‘life hypothesis’ as highly improbable. To reject the life hypothesis, we need more data from future missions.”
Using the newly developed model, the team gave a set of conditions, including dihydrogen concentration and different temperatures to understand if microbes would grow. They also looked at what amount of methane would be emitted if there was a hypothetical microbe population on Enceladus. “In summary, not only could we evaluate whether Cassini’s observations are compatible with an environment habitable for life, but we could also make quantitative predictions about observations to be expected, should methanogenesis actually occur at Enceladus’ seafloor,” Prof. Ferrière explained.
What other processes could have produced the methane?
The team writes that methane could be formed by the chemical breakdown of organic matter present in Enceladus’ core. Hydrothermal processes could help the formation of carbon dioxide and methane. On Earth, hydrothermal vents on seafloors are known to release methane, but this happens at a very slow rate. Ferrière explained that this hypothesis is plausible but only if Enceladus was formed through the accretion of organic-rich material from comets.
The results suggest that methane production from hydrothermal vents is not sufficient to explain the high methane concentration detected by Cassini in the plumes. An additional amount of methane produced via biological methanogenesis could match Cassini’s observations. “Searching for such microbes at Enceladus’ seafloor would require extremely challenging deep-dive missions that are not in sight for several decades,” concludes Prof. Ferrière.
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