An international scientific team led by Antonin Affholder from the University of Paris in France wanted to know whether the gases emanating from Saturn’s moon Enceladus are likely of chemical or biological origin. During close fly-bys of the moon ending in 2015, the Cassini spacecraft detected hydrogen and methane, among other gases, which implied the presence of hydrothermal activity in the liquid water ocean beneath Enceladus’s icy surface. Could these be analogous to hydrothermal vents like the “black smokers” on Earth, which are a haven for biology? Microbes living at these vents produce methane (and water) from hydrogen and carbon dioxide gas as part of their natural metabolic reactions.
The science team used a method known as Bayesian statistics, where probability is expressed as a degree of belief based on prior obtained data and insights. They took mathematical models of known geophysical, geochemical, and biological processes, and included them in their statistical approach to quantify the plausibility of different hypotheses about Enceladus.
Methane can be an indicator of biology, but it can also form through an abiotic reaction that most commonly occurs in Earth’s oceanic crust, called serpentinization, which produces methane when water reacts with certain rock types.
Affholder’s results showed that the methane concentrations measured in Enceladus’s gaseous plume are too high to be solely due to serpentinization. In fact, they concluded that the likeliest explanation is biology. Otherwise, to explain the high methane levels, an unknown abiotic process would have to be invoked.
This reminds me of the controversy over phosphine at Venus, which also was presented as being explainable only by biology or an unknown chemical process. While the detection of methane and hydrogen in the Enceladus plume is undebatable (contrary to the phosphine detection at Venus), this study is still inconclusive, since we aren’t certain whether life is even possible inside Enceladus.
The moon’s ice-covered ocean may well be habitable now (I actually expect that), but if life requires Darwin’s “little pond” on a solid planetary surface to arise in the first place, it would not have developed on Enceladus. In general, environmental constraints for the origin of life are likely much more restrictive then for its presence. In other words, there may be many uninhabited habitable places in the galaxy.
As long as we haven’t resolved this central question, I’m not certain whether statistical analyses like Affholder’s will really move us forward. But the question of life beneath the icy surface of Enceladus needs to be investigated. Any anomaly that we can’t readily explain needs to be looked at more closely. We might even have a stroke of luck. Let’s say the authors are correct about methanogenesis. If a follow-up mission to Saturn detects biology on Enceladus, it would tell us that hydrothermal vents are one place that life can originate.