Could the methane we’ve detected on Enceladus be signs of life? NASA/JPL-Caltech/Space Science Institute

Microbes that produce methane may already be living on Enceladus, a moon of Saturn which is tipped to host life because it boasts a liquid water ocean beneath a crust of solid ice, and strange atmospheric plumes of water. That’s the implication of research showing that an earthbound organism which also produces methane can happily survive in conditions known to exist on Enceladus, from observations by the Cassini space probe before its mission ended last year.

Isolated from deep sea vents almost 1000 metres deep in the Okinawa Trough off Japan, Methanothermococcus okinawensis was subjected to gruelling physical and chemical conditions found on Enceladus for more than five years.

This microbe, called a methanogenic archaeon, survives without oxygen by combining hydrogen and carbon dioxide – both observed in Enceladus’s atmosphere – to make the energy it needs, emitting methane as a waste product. Cassini detected traces of methane in Enceladus’s plumes, and there’s a chance that some of it may have come from this kind of microbe.


“The conditions we mimicked in the lab are as close as possible to those inferred from Cassini on Enceladus,” says Simon Rittmann at the University of Vienna in Austria, who led the investigations.

Rittmann subjected the microbe to various combinations of gases found on Enceladus, and found that it was always able to survive when provided with the moon’s levels of hydrogen and carbon dioxide. It was still able to thrive at temperatures and pressures likely found in Enceladus’s oceans, ranging respectively from 0 to 90 degrees Celsius, and up to 50 Earth atmospheres.

Life on the seafloor

Rittmann’s team also computed how much hydrogen would be produced by a breakdown of olivine minerals – which are predicted to make up the moon’s solid core – under a range of likely geological conditions on Enceladus. They found these minerals could break down chemically to produce enough hydrogen for methanogens to thrive.

The best environment for them is likely to be the seafloor. “There, you have contact with rock and minerals, pressures of around 50 atmospheres and temperatures most likely a bit higher than 0 degrees Celsius,” says Rittmann.

“This [team] has taken the first step to showing experimentally that methanogens can indeed live in the conditions expected on Enceladus,” says Chris McKay at NASA Ames Research Center in Moffett Field, California.

Rittmann says he hopes there will be future missions to Enceladus to explore further for signs of life. He says a probe fitted with a mass spectrometer would be able to detect carbon isotope ratios unique to living organisms, as well as other potential “biomarkers” of methanogens, including lipids and hydrocarbons.

“If we find life on Enceladus, it is not likely to be very Earth-like, unless the origin of these life forms is from a common source outside the solar system, which is highly unlikely,” says Hunter Waite at the Southwest Research Institute in Texas. Proof of concept on Earth is interesting, he says, but there is no substitute for finding and studying a methanogenic organism in the unique environment of Enceladus.

Journal reference: Nature Communications, DOI: 10.1038/s41467-018-02876-y

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