Short lifespans of serpentinization in the rocky core of Enceladus: Implications for hydrogen production

The discovery of a liquid ocean on Saturn's small moon Enceladus and evidence of modern hydrothermal activity provide an unexpected new environment in which to expand the search for life. However, as with the age of the moons themselves, the age of the liquid ocean and any hydrothermal activity therein remains an area of debate. Based on physical and chemical observations from the Cassini mission we can apply known mineral dissolution rates, estimated water-rock ratios from Enceladus' density, and variable water flow rates within the rocky core to constrain durations of active serpentinization. On this basis we developed a 1-D reactive transport model to compare the effect of initial olivine percentage, grain size, temperature, and flow rate on timespans of primary olivine alteration in a rocky core the size and density of Enceladus'. In most cases, olivine alteration and precipitation of hydrous secondary minerals results in a water-limited alteration regime. An alteration front that propagates in the direction of water flow then controls the overall rate of olivine alteration. Of the parameters explored, high initial olivine percentages and slow fluid flow rates were the strongest predictors of long serpentinization times, while temperature and grain size had a smaller effect. The annual global H2 production rate in all model cases (> 1 × 1012 mol yr−1) is several orders of magnitude greater than the minimum H2 release rate calculated from the observed H2 in Enceladus' plume (1 × 109 mol yr−1), suggesting that any ongoing active serpentinization processes in the core are likely nearing completion. The longest timescales indicate the potential for olivine alteration and H2 production for up to ~75 Myr, consistent with weathering rates of terrestrial peridotite massifs. If the H2 produced from Enceladus is sourced from primary mineral alteration, these results suggest that hydrothermal activity in the core of Enceladus may have developed only very recently – even as recent as within the past 100 Myr.