The search for methane on other planets is a key element in the search for life on other planets because one cause of the carbon gas is the decaying of organic matter. Unfortunately for life searchers, the most common cause of methane is abiotic or inorganic — either because of magmatic (volcanic) processes or via water-rock reactions that occur at low temperatures and pressures. That’s why the discovery of giant water plumes containing methane erupting from the surface of Saturn’s moon Enceladus by Cassini spacecraft generated mixed reactions from scientists depending on whether they saw their glass of possible life half empty or half full. The level of that glass just increased with the release of a new study which determined that no known process could generate as much methane as that erupting in plumes on Enceladus. Is it life … or something else?
Cassini image of Enceladus backlit by the sun showing the fountain-like sources of the fine spray of material over the south polar region. (NASA)
“Here we use a Bayesian statistical approach to quantify the probability that methanogenesis (biotic methane production) might explain the escape rates of molecular hydrogen and methane in Enceladus’s plume, as measured by Cassini instruments”.
Scientists at the University of Arizona and Paris Sciences & Lettres University knew the best way to determine if the methane was being generated by Earthlike microbes eating dihydrogen and producing methane in such huge quantities would be to be to go where they are – on the seafloor of one of the water bodies on Enceladus. Since that’s not currently possible, they did the next best thing — develop mathematical models to calculate the probability of the different known processes of methane generation, both biological and abiotic, captured in the Cassini data. The researchers used the Bayesian statistics approach which expresses probability as a degree of belief in an event.
“We find that the observed escape rates (1) cannot be explained solely by the abiotic alteration of the rocky core by serpentinization; (2) are compatible with the hypothesis of habitable conditions for methanogens; and (3) score the highest likelihood under the hypothesis of methanogenesis, assuming that the probability of life emerging is high enough”.
That’s as close as the authors of the study, published in the journal Nature Astronomy, will get to saying, “We’re not saying it’s alien life, but it’s alien life”. In the University of Arizona press release, Régis Ferrière, an associate professor in the University of Arizona Department of Ecology and Evolutionary Biology and one of the study’s two lead authors, explains that the results show that even the highest possible estimate of abiotic methane production based on known hydrothermal chemistry is far from sufficient to explain the methane concentration measured in the plumes. Add it together with the probability of biological methanogenesis and the model shows the combination could produce enough methane to match Cassini’s observations. In other words, the probability of life on Enceladus at some point in time just made the glass more than half full.
An artist’s concept of Saturn’s rings and major icy moons. (NASA)
Then again, it’s just a model. Missions to Enceladus which would settle the argument have been proposed since 2006, with half already canceled and half under study – but none funded and planned. Will this study push the Enceladus mission needle to the ‘planned, funded and scheduled’ side anytime soon? We can only hope.
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