Methane in crest of Saturn’s moon Enceladus: Possible indications of life?

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An unknown methane-producing process is probably going to add the hidden ocean beneath the icy shell of Saturn’s moon Enceladus, suggests a replacement study published in Nature Astronomy by scientists at the University of Arizona and Paris Sciences & Lettres University.

Giant water plumes erupting from Enceladus have long fascinated scientists and therefore the public alike, inspiring research and speculation about the vast ocean that’s believed to be sandwiched between the moon’s rocky core and its icy shell. Flying through the plumes and sampling their chemical makeup, the Cassini spacecraft detected a comparatively high concentration of certain molecules related to hydrothermal vents on the rock bottom of Earth’s oceans, specifically dihydrogen, methane, and CO2 . the quantity of methane found within the plumes was particularly unexpected.

“We wanted to know: Could Earthlike microbes that ‘eat’ the dihydrogen and produce methane explain the surprisingly great deal of methane detected by Cassini?” said Regis Ferriere, a professor within the University of Arizona Department of Ecology and Evolutionary Biology and one among the study’s two lead authors. “Searching for such microbes, referred to as methanogens, at Enceladus’ seafloor would require extremely challenging deep-dive missions that aren’t in view for several decades.”

Ferriere and his team took a special, easier route: They constructed mathematical models to calculate the probability that different processes, including biological methanogenesis, might explain the Cassini data.

The authors applied new mathematical models that combine geochemistry and microbial ecology to research Cassini plume data and model the possible processes that might best explain the observations. They conclude that Cassini’s data are consistent either with microbial hydrothermal vent activity or with processes that do not involve life forms but are different from those known to occur on Earth.

On Earth, hydrothermal activity occurs when cold seawater seeps into the ocean bottom, circulates through the underlying rock, and passes accessible a heat source, like a magma chamber, before spewing out into the water again through hydrothermal vents. On Earth, methane is often produced through hydrothermal activity, but at a slow rate. Most of the assembly is thanks to microorganisms that harness the chemical disequilibrium of hydrothermally produced dihydrogen as a source of energy and produce methane from CO2 during a process called methanogenesis.

The team checked out Enceladus’ plume composition because of the outcome of several chemical and physical processes happening within the moon’s interior. First, the researchers assessed what hydrothermal production of dihydrogen would best fit Cassini’s observations and whether this product could provide enough “food” to sustain a population of Earthlike hydrogenotrophic methanogens. to try to do that, they developed a model for the population dynamics of a hypothetical hydrogenotrophic methanogen, whose thermal and energetic niche was modeled after known strains from Earth.

The authors then ran the model to ascertain whether a given set of chemical conditions, like the dihydrogen concentration within the hydrothermal fluid and temperature, would offer an appropriate environment for these microbes to grow. They also checked out what effect a hypothetical microbe population would wear its environment — for instance, on the escape rates of dihydrogen and methane within the plume.

“In summary, not only could we evaluate whether Cassini’s observations are compatible with an environment habitable for all times, but we could also make quantitative predictions about observations to be expected, should methanogenesis actually occur at Enceladus’ seafloor,” Ferriere explained.

The results suggest that even the very best possible estimate of abiotic methane production — or methane production without biological aid — supported known hydrothermal chemistry is way from sufficient to elucidate the methane concentration measured within the plumes. Adding biological methanogenesis to the combination, however, could produce enough methane to match Cassini’s observations.

“Obviously, we aren’t concluding that life exists in Enceladus’ ocean,” Ferriere said. “Rather, we wanted to know how likely it might be that Enceladus’ hydrothermal vents might be habitable to Earthlike microorganisms. Very likely, the Cassini data tell us, consistent with our models.

“And biological methanogenesis appears to be compatible with the info. In other words, we will not discard the ‘life hypothesis’ as highly improbable. To reject the life hypothesis, we’d like more data from future missions,” he added.

The authors hope their paper provides guidance for studies aimed toward better understanding the observations made by Cassini which encourages research to elucidate the abiotic processes that would produce enough methane to elucidate the info.

For example, methane could come from the chemical breakdown of primordial organic matter which will be present in Enceladus’ core which might partially become dihydrogen, methane, and CO2 through the hydrothermal process. This hypothesis is extremely plausible if it seems that Enceladus formed through the accretion of organic-rich material supplied by comets, Ferriere explained.

“It partly boils right down to how probable we believe different hypotheses are to start with,” he said. “For example, if we deem the probability of life in Enceladus to be extremely low, then such alternative abiotic mechanisms become far more likely, albeit they’re very alien compared to what we all know here on Earth.”

According to the authors, a really promising advance of the paper lies in its methodology, because it isn’t limited to specific systems like interior oceans of icy moons and paves the thanks to affecting chemical data from planets outside the system as they become available within the coming decades.

A full list of authors and funding information are often found within the paper, “Bayesian analysis of Enceladus’s plume data to assess methanogenesis,” within the July 7 issue of Nature Astronomy.

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