The Carbonate Geochemistry of Enceladus' Ocean

The plume composition at Enceladus contains clues about conditions and processes in the interior. We present new geochemical interpretations of Cassini mass spectrometry data from the plume gas and salt-rich ice grains. It is found that self-consistency between the data sets can be achieved with a derived range of 10(-4.6) to 10(-3.2) for the activity of CO2 in Enceladus' ocean. This range is compatible with long-term buffering by reduced or oxidized seafloor rocks containing quartz, talc, and carbonate minerals in the MgO-FeO-SiO2-CO2-H2O system. Reaction path modeling shows that these types of rocks can be produced from accreted CO2-rich fluids reacting with hydrous chondritic rocks over an intermediate regime of carbonation. These results, together with previous findings of silica and H-2 at Enceladus, support the hypothesis of a heterogeneous structure for the rocky core (carbonated upper layer, serpentinized interior), which provides a geochemical gradient for habitability.