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

The Cassini spacecraft demonstrated that Saturn's small moon Enceladus may harbor hydrothermal activity. In particular, molecular hydrogen production could result from water-rock interactions in a tidally heated, water-filled porous rocky core. The lifetime of such reactions is key to assess the habitability potential of Enceladus and to constrain plausible durations of the active stage in a context where the evolution of the moon is debated. Although it has recently been suggested that the serpentinization timescale does not exceed a few hundred million years, this estimation was based on assumptions regarding silicate dissolution kinetics that are prone to overestimate the actual reactivity of primary silicates. Here, we investigated plausible rate-limiting mechanisms governing fluid-rock interactions that could delay the completion of Enceladus' core serpentinization. In particular, we considered the impact of (a) various secondary mineral assemblages on the Gibbs free energy of Fe-bearing silicate dissolution and associated dissolution rates; (b) rate-laws alternative to the transition state theory; (c) diffusion in nanoporous secondary assemblages; (d) slow water supply. Overall, our results confirm that serpentinization timescales never exceed 500 Myr, and indicate that fluid flow ultimately sets the tempo for serpentinization. Only unreasonable grain sizes in Enceladus' core (>1 m) or unexpectedly low diffusivity of secondary coatings covering primary silicates would be consistent with serpentinization durations of several billion years. We thus suggest that either the hydrothermal activity has developed recently on Enceladus, or alternative processes (pyrolysis of insoluble organic matter, microbial activity) must be tested to explain the observed H-2 flux in Enceladus' plume.