Bayesian analysis of Enceladus’s plume data to assess methanogenesis

Abstract

Observations from NASA's Cassini spacecraft established that Saturn's moon Enceladus has an internal liquid ocean. Analysis of a plume of ocean material ejected into space suggests alkaline hydrothermal vents on Enceladus' seafloor. On Earth, such deep-sea vents harbor microbial ecosystems rich in methanogenic archaea. 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' plume, as measured by Cassini instruments. We find that the observed escape rates (i) cannot be explained solely by the abiotic alteration of the rocky core by serpentinization; (ii) are compatible with the hypothesis of habitable conditions for methanogens; (iii) score the highest likelihood under the hypothesis of methanogenesis, assumed the probability of life emerging is high enough. If the probability of life emerging on Enceladus is low, the Cassini measurements are consistent with habitable yet uninhabited hydrothermal vents and point to unknown sources of methane (e.g., primordial methane) awaiting to be discovered by future missions.

Theoretical Considerations on the Characteristic Timescales of Hydrogen Generation by Serpentinization Reactions on Enceladus

“…The presence of heavy organics (Postberg et al., 2018) raises the possibility that their pyrolysis could produce the present time H 2 (Sotin & Reynard, 2021). It would also explain at the same time similar amounts of CH 4 and H 2 in the plumes, which are difficult to explain by serpentinization without invoking contribution of alive microorganisms (Affholder et al., 2021). In that case, a low porosity inner‐core of Enceladus made of an assemblage of hydrated silicates and organic matter, and an outer core made of porous hydrated silicates required for tidal heating could explain the Cassini data.…”

Enceladus as a Potential Niche for Methanogens and Estimation of Its Biomass

“…Affholder et al [45] have shown that combined biotic and abiotic methanogenesis could explain the composition in Enceladus' plume, and different authors have already estimated the possible concentration of cells in the Saturnian moon. Using the geothermal energy flux ratio and scaling it to Earth's, Porco et al [46] assumed that the relation of biomass and geothermal flux is the same in both bodies and estimated the biomass in Enceladus to be 10 5 cells/ml.…”

Searching for Life, Mindful of Lyfe’s Possibilities

“…By way of example, a recent study applied a probabilistic framework to data collected by the Cassini mission and claimed that Enceladus’ methane was most probably due to life [ 12 ]. In this study, “life” refers to direct analogs of Earth’s prokaryotic methanogens.…”