  Observational data from the Cassini spacecraft are used to obtain a chemical
model of ocean water on Enceladus. The 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 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 owing to buffering by clays and carbonates on the ocean floor.
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 role depending on the
efficiency of mixing processes in the ocean. Serpentinization 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 high pH also suggests that the delivery of oxidants from the
surface to the ocean has not been significant, and the rocky core did not
experience partial melting and igneous differentiation. On the other hand, the
pH is compatible with life as we know it; life on Earth may have begun under
similar conditions, and serpentinites on Earth support microbial communities
that are centered on H2 that is provided by water-rock reactions.
