Enceladus’s global ocean lies under an ice crust and above a rocky core where tidal dissipation18,19 is suspected to drive hydrothermal activity4,5. There are several lines of evidence describing how volatile5 and dissolved materials3,4,11 from the rocky core are either emitted by the plume in the gas phase or incorporated into ice particles, respectively.
Cassini’s Cosmic Dust Analyzer (CDA) recorded time-of-flight (ToF) mass spectra with a mass resolution of m/Δm ≈ 10–50 (refs. 10,20) for cations generated by high-velocity impacts of individual grains onto the instrument’s rhodium target. The E-ring of Saturn is formed by ice grains escaping Enceladus’s plume into orbits around Saturn21, and hence the analysis of these grains by CDA provides important insights into the composition of the subsurface ocean—including a rich variety of organic compounds6,7,8,9—with much better statistics compared with data from the rare occasions when Cassini traversed the plume itself.
In a previous analysis it was shown that a fraction of these ice grains, called Type 3 particles, contain salts at significantly higher concentrations than all other ice grains in the plume and the E-ring3,11. By co-addition of spectra from 107 individual detections in the E-ring, average salt concentrations of 0.5–2.0% by weight (around 0.07–0.30 M) have been deduced for Type 3 grains, with NaCl, NaHCO3, Na2CO3 and KCl as the most abundant constituents. Because it was inferred that these grains are derived from aerosolized ocean water3, the salt chemistry of the plume and E-ring grains would reflect that of the ocean.
Phosphorus is an element essential for planetary habitability12,13,14, but to date it has not been detected in an ocean beyond Earth. Previous geochemical modelling suggested that phosphate might be scarce in the ocean of Enceladus and other icy ocean worlds15,16. However, more recent modelling of mineral solubilities in Enceladus’s ocean suggests that phosphate could be relatively abundant (roughly 10−7–10−2 M), depending on the compositional characteristics of the ocean such as its pH and carbonate content, as well as on attainment of certain chemical equilibria between ocean water and seafloor alteration minerals17.