The pH of Enceladus’ ocean

Abstract

Saturn's icy moon, Enceladus, is a geologically active waterworld. The prevailing paradigm is that there is a subsurface ocean that erupts to the surface, which leads to the formation of a plume of vapor and ice above the south pole. The chemical composition of the ocean is just beginning to be understood, but is of profound geochemical and astrobiological interest. Here, we address the most fundamental question about the ocean's chemistry by determining its pH, using a thermodynamic model of carbonate speciation. Observational data from the Cassini spacecraft are used to obtain a chemical model of ocean water on Enceladus, the first such model for another planet/moon that is based on empirical evidence. The abundance of CO2 in the plume gas, as measured by the Ion and Neutral Mass Spectrometer onboard Cassini, is essential to elucidating the pH. The geochemical model indicates that Enceladus' ocean is a Na-Cl-CO3 solution with an alkaline pH of ~11-12. The dominance of aqueous NaCl is a geochemical feature that Enceladus' ocean shares with terrestrial seawater, but the ubiquity of dissolved Na2CO3 suggests that soda lakes are more analogous to the Enceladus ocean. The high pH implies that the hydroxide ion should be relatively abundant, while divalent metals should be present at low concentrations in ocean water owing to buffering by clays and carbonates on the ocean floor. In addition, carboxyl groups in dissolved organic species (such as possible biomolecules) would be negatively charged, whereas amino groups would exist predominately in the neutral form. Knowledge of the pH dramatically improves our understanding of geochemical processes in Enceladus' ocean. In particular, the high pH is interpreted to be a key consequence of serpentinization of chondritic rock, as predicted by prior geochemical reaction path models, although degassing of CO2 from the ocean may also play a 3 role depending on the efficiency of mixing processes in the ocean. Serpentinization inevitably leads to the generation of H2, a geochemical fuel that can support both abiotic and biological synthesis of organic molecules such as those that have been detected in Enceladus' plume.Serpentinization and H2 generation should have occurred on Enceladus, like on the parent bodies of aqueously altered meteorites; but it is unknown whether these critical processes are still taking place, or if Enceladus' rocky core has been completely altered by past hydrothermal activity. The presence of native H2 in the plume would provide strong evidence for contemporary aqueous alteration. The high pH also suggests that the delivery of oxidants from the surface to the ocean has been sporadic, and the rocky core did not experience partial melting and igneous differentiation. On the other hand, the deduced pH is completely compatible with life as we know it; indeed, life on Earth may have begun under similar conditions, and terrestrial serpentinites support thriving microbial communities that are centered on H2 that is provided by water-rock reactions. T...

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