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

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

“…Overall, considering a serpentinization regime that is controlled either by the water velocity in the core of Enceladus or by aqueous diffusion in nanoporous secondary minerals provides serpentinization timespans that are at least two orders or magnitude greater than those estimated for surface‐controlled reactions. Nonetheless, these results suggest that hydrothermal activity in the core developed only recently and no earlier than 500 Myr ago, in agreement with the results of (Zandanel et al., 2021).…”

“…Considering the slowest fluid velocity suggested by (Choblet et al., 2017), the lifetime of serpentinization ranges between <1 and ∼500 Myr, depending on the porosity, the Fe content in olivine and chemical reaction considered (Figure 6). Note that these values are in excellent agreement with the outputs of 1D‐reactive transport simulations of (Zandanel et al., 2021) conducted with a similar set of input parameters for fluid flow rate and core porosity. Therefore, this result independently confirms that the rate‐limiting step in their corresponding simulations is the water supply, consistent with the sharp serpentinization fronts reported in their Figure 3.…”

“…However, this strategy is not devoid of risks, because it does not account for the mechanistic origins of the “field‐lab discrepancy”. With this in mind, one can wonder the extent to which the short timescales of molecular hydrogen generation by serpentinization reactions on Enceladus suggested in previous studies (e.g., Zandanel et al., 2021) could not originate from the use of TST‐based rate laws, which do not account for the rate‐controlling role exerted by the spontaneous modifications of reacted mineral surface (e.g., etch pit nucleation or development of passivating layers), ultimately leading to underestimated timescales of serpentinization. Therefore, a detailed kinetic analysis is required to evaluate if the conclusions drawn in such previous studies may be a consequence of the assumptions they made to describe olivine dissolution kinetics.…”

“…Based on 1D‐reactive transport simulations conducted with the PHREEQC code (Parkhurst & Appelo, 2013), (Zandanel et al., 2021) recently suggested short timescales for H 2 generation, never exceeding 300 Myr at most. Their simulations supposed that olivine dissolution was the rate‐limiting step of the serpentinization process (and therefore, of the associated production of H 2 ) and relied on parameters taken from the literature for fluid flow, olivine grain size, core porosity and olivine dissolution kinetics.…”

“…A stepwise approach is described below, starting from the well-known "lifetime diagram" approach developed by (Olsen & Rimstidt, 2007). We show that although this approach may seem incomplete (as it does not account for the impact of the Gibbs free energy of reaction with respect to olivine (or orthopyroxene) dissolution), practically, it actually describes the intrinsic reactivity of primary minerals following the same rates as reactive transport simulations where secondary phases are supposed to precipitate at equilibrium (as in the work of [Zandanel et al, 2021]). Then, based on assumptions consistent with the literature regarding the fluid chemistry of Enceladus, we screen alternative rate-limiting mechanisms (and corresponding rate laws) such as the impact of (a) etch pit nucleation; (b) surface layers; (c) diffusion in nanoporous secondary assemblages; (d) fluid flow.…”