Chemical fractionation in Enceladus’ plume driven by dynamic exsolution and condensation

Abstract:

Plumes erupting at the south pole of Enceladus and sampled by the Cassini spacecraft provide a unique window into the composition and habitability of Enceladus’ subsurface ocean. However, chemical fractionation during plume eruption, which creates compositional differences between the plume and ocean, has not been fully considered. We construct a model to derive ocean gas composition from plume measurements by modeling fractionation during two processes: (1) dynamic gas exsolution at the surface of the ocean and (2) dynamic condensation of water vapor as the plumes travel upward through icy fissures. We use a thin-film model of gas exsolution, and adopt a model of plume water vapor condensation developed in a previous study.

We find that exsolution enriches the relative concentrations of gases in the plume in the following order, according to their rates of exsolution: H₂O >> NH₃ >> CO₂ > CH₄ ≈ H₂. Conversely, as previous studies have observed, condensation removes water vapor from the plume, enhancing the concentrations of the other gases, which do not condense. Our model predicts higher concentrations of dissolved gases (CO₂, H₂, CH₄, NH₃) in the ocean compared to studies that accounted for condensation but not dynamic exsolution. Our predicted higher concentrations of H₂ and CO₂ result in a greater capacity for methanogenic life in the ocean due to more available chemical energy. Due to higher predicted concentrations of CO₂, we estimate an ocean pH between ~8-10, which is lower than previous estimates of ~9-11. Ocean pH in turn will have a significant effect on speciation in the ocean. For example, a lower pH corresponds to a greater availability of free protons (H⁺), and therefore implies a higher ratio of NH₄⁺/NH₃, which is relevant for possible life. In summary, gas exsolution and water vapor condensation have significant fractionation effects on Enceladus’ plume composition, and our modeling results predict a lower pH and higher concentrations of biologically useful gases in the subsurface ocean than previous work.