Germs on Enceladus? Methane on Saturn’s moon cannot be explained by known geochemical processes

An unknown process of methane production is likely at work in the ocean hidden under the icy shell of Saturn’s moon Enceladus, suggests a new study published in Nature Astronomy by scientists from the University of Arizona and the University of Paris Sciences & Lettres.

The giant plumes of water rising from Enceladus have long fascinated scientists and the public, inspiring research and speculation about the vast ocean that would be sandwiched between the rocky core of the moon and its icy shell.

By hovering over the plumes and sampling their chemical composition, the Cassini spacecraft detected a relatively high concentration of certain molecules associated with hydrothermal vents at the bottom of Earth’s oceans, particularly dihydrogen, methane, and carbon dioxide. The amount of methane found in the plumes was a surprise – as it cannot be explained by known geochemical processes.

This means that an unknown process of methane production is likely at work in the Hidden Ocean. Which could have implications for life there, like the microbes – the methanogens – that “eat” hydrogen and produce methane.

To try to gain insight, a team built mathematical models to try to explain what Cassini detected – including the possibility of biological methanogenesis.

They conclude that Cassini’s data is consistent either with the activity of microbial hydrothermal vents or with processes that do not involve life forms but are different from those known on Earth.

Graphic representation of a sectional view of the moon of Saturn Enceladus which describes the possible hydrothermal activity that could take place on and under the seabed of the moon’s underground ocean, based on the results of the Cassini mission of The NASA. NASA / JPL-Caltech

On Earth, hydrothermal activity occurs when cold seawater seeps into the ocean floor, flows through the underlying rock, and passes near a heat source, such as a magma chamber, before spitting back into the water through hydrothermal vents. On Earth, methane can be produced by hydrothermal activity, but at a slow rate. Most of the production is from microorganisms that harness the chemical imbalance of hydrothermally produced hydrogen as an energy source and produce methane from carbon dioxide in a process called methanogenesis.

The team examined the composition of the Enceladus plume as the end result of several chemical and physical processes taking place inside the moon.

First, the researchers evaluated which hydrothermal production of dihydrogen would best match Cassini’s observations, and whether this production could provide enough “food” to support a population of Earth-like hydrogenotrophic methanogens. To do this, they developed a model for the population dynamics of a hypothetical hydrogenotrophic methanogen, whose thermal and energy niche was modeled on known strains of Earth.

The authors then ran the model to see if a given set of chemical conditions, such as the concentration of hydrogen in the hydrothermal fluid and temperature would provide an environment conducive to the growth of these microbes. They also examined the effect of a hypothetical population of microbes on its environment, for example on the rates of hydrogen and methane leakage in the plume.

The results suggest that even the highest possible estimate of abiotic methane production – or methane production without biological aid – based on known hydrothermal chemistry is far from sufficient to explain the concentration of methane measured in plumes. Adding biological methanogenesis to the mix, however, could produce enough methane to match Cassini’s observations.

About Alma Ackerman

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