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 that alkaline hydrothermal vents are present on Enceladus’s seafloor. On Earth, such deep-sea vents harbour 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’s plume, as measured by Cassini instruments. 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. 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 (for example, primordial methane) awaiting discovery by future missions.

美国宇航局卡西尼号宇宙飞船的观测证实土星的卫星土卫二内部有一个液态海洋。对喷射到太空的海洋物质羽流的分析表明，土卫二的海底存在碱性热液喷口。在地球上，这样的深海通风口拥有富含产甲烷古细菌的微生物生态系统。在这里，我们使用贝叶斯统计方法来量化产甲烷（生物甲烷产生）可能解释土卫二羽流中分子氢和甲烷逃逸率的概率，如卡西尼仪器测量的那样。我们发现观察到的逃逸率 (1) 不能仅仅通过蛇纹石化对岩石核心的非生物改变来解释；(2) 符合产甲烷菌宜居条件的假设；(3) 在产甲烷假设下得分最高的可能性，假设生命出现的概率足够高。如果土卫二上出现生命的概率很低，卡西尼号的测量结果与可居住但无人居住的热液喷口一致，并指出等待未来任务发现的未知甲烷来源（例如原始甲烷）。