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.

The history of the Earth has been marked by major ecological transitions, driven by metabolic innovation, that radically reshaped the composition of the oceans and atmosphere. The nature and magnitude of the earliest transitions, hundreds of million years before photosynthesis evolved, remain poorly understood. Using a novel ecosystem-planetary model, we find that pre-photosynthetic methane-cycling microbial ecosystems are much less productive than previously thought. In spite of their low productivity, the evolution of methanogenic metabolisms strongly modifies the atmospheric composition, leading to a warmer but less resilient climate. As the abiotic carbon cycle responds, further metabolic evolution (anaerobic methanotrophy) may feed back to the atmosphere and destabilize the climate, triggering a transient global glaciation. Although early metabolic evolution may cause strong climatic instability, a low CO:CH 4 atmospheric ratio emerges as a robust signature of simple methane-cycling ecosystems on a globally reduced planet such as the late Hadean/early Archean Earth.