Methanogenic Life at Enceladus Seafloor as a Possible Explanation for Cassini’s Observations: a Bayesian Analysis of a Mechanistic Ecological-Geochemical Model

Abstract:

Enceladus has attracted much interest in the search for life and habitable environments beyond Earth (Waite et al 2017, Taubner et al 2018). Data from the Cassini space mission revealed the existence of a global ocean of circulating water underneath Enceladus' ice shell (Waite et al 2009, Postberg et al 2009, Travis and Schubert 2015, Choblet et al 2017). The chemical analysis of large plumes of oceanic material ejected into space at a polar active fault system supports the existence of Earth-like alkaline hydrothermal vents at Enceladus seafloor (Waite et al 2017). On Earth, alkaline vents harbor microbial ecosystems rich in methanogenic archea (Schrenk et al 2004), raising the possibility that similar microorganisms dwell in Enceladus' deep ocean. Here we use Enceladus' plume composition inferred from the Cassini data to evaluate the likelihood of active methanogenic life in Enceladus-like hydrothermal environments. To this end, we use a Bayesian framework in which we couple a new ecological model of Earth-like hydrogenotrophic, thermophilic methanogens (Sauterey et al 2020) with a one-dimensional model of circulation. We find that (1) the observed escape rate of molecular hydrogen in the plume is indicative of favorable conditions for methanogens at the bottom of Enceladus' ocean, (2) other markers such as the rate of methane production are needed to evaluate the likelihood of their actual existence, and (3) the methane production rate expected from methanogenic communities matches the observed rate of escape with high likelihood, while serpentinization waters chemistry alone fails to reproduce the observations. We conclude that methanogens are a possible explanation for the observed composition of Enceladus’ plume, given that primordial sources of methane (which are poorly constrained) are not modelled. New data from future space missions will be critical to identify abiotic sources of methane that can explain Cassini's observations as well as methanogenic biological activity does.