The Chemistry of Enceladus’ Ocean from a Convergence of Cassini Data and Theoretical Geochemistry

Introduction: Saturn’s icy satellite Enceladus is strongly suspected of possessing a subsurface ocean of liquid water below its geologically active south polar region [1-5]. The ocean is thought to be the source of chemicals in Enceladus’ plume [3,4]. The composition of the ocean is of great interest, as it contains key clues to the geochemical evolution of Enceladus [6], it sets the boundary conditions for possible living processes inside Enceladus [7], and it can serve as a model for understanding the potential ocean chemistries and astrobiological potential of other icy worlds [8] that may not be erupting free samples into space. With the end of the Cassini mission in sight, we seek to maximize the existing data to constrain the geochemistry of Enceladus’ ocean as much as possible to help prepare for the next generation of Enceladus exploration. There are significant gaps in our understanding of the chemistry of Enceladus’ ocean. [9] developed a theoretical model of chemical equilibrium between water and chondritic rock, and predicted the pH and concentrations of major chemical species in the ocean. However, this model was developed before the chemistry of the plume was characterized in detail [3,4], so the model could not be constrained by observational data. Here, we use Cassini CDA and INMS data to develop the first top-down (observationally constrained) model of the chemical composition of Enceladus’ ocean [10]. We then take a bottom-up theoretical approach in an attempt to explain the derived composition in terms of simple geochemical equilibria. Top-down model: Consistent with prior work [14], we assume that the plume gases and ice grains are derived from a subsurface ocean. In this model, the gases are generated by the degassing of ocean water at low-pressure interfaces where the tiger stripe fractures intersect the ocean, and the salt-rich grains represent ocean water that is flash-frozen during the eruption process. We focus on the Na-Cl-C-N-O-H chemical system, and calculate the composition of ocean water that would yield plume gas and ice grain compositions consistent with those reported by [11] and [3], respectively. The composition of the plume gas is updated from [4] based on more recent, slower flybys that provide data that more closely reflect the unmodified composition of the plume. We account for a distillation effect that concentrates volatile species in the plume gas relative to the source region, as a consequence of the removal of water vapor by condensation while the warm vapor travels through colder fractures [2, 10]. These considerations allow us to transform the observational data into a set of constraints that can be coupled to geochemical speciation calculations that are performed using the Geochemist’s Workbench [12]. Top-down results: From the measured composition of the plume gas [11], we set an upper limit of ~0.01 for the activity of NH3, and we find that the activity of CO2 should fall in the range 10−11.2-10−7.3. These values are not more specific because of uncertainties in modeling the distillation effect [10]. Nevertheless, they provide valuable insights into the chemistry of Enceladus’ ocean. The NH3 activity is approximately equal to its molality (m). The upper limit is interesting because, based on observations of comets [13], we might expect the concentration of primordial NH3 to be ~0.1-1 m, or even higher if the eutectic mixture is present. It is possible that Enceladus accreted “warm” ices that were depleted in NH3, but this would be inconsistent with the elevated D/H in water [4]. Instead, the “missing” NH3 may have been removed by past outgassing, hydrothermally oxidized to N2 [6], or incorporated into organic or biomolecules [4,7]. By constraining the CO2 activity, we can estimate the carbonate speciation and pH of the ocean [10].