Phosphates Reveal High pH Ocean Water on Enceladus

Abstract

Enceladus offers our best opportunity for exploring the chemistry of an ocean on another world. Here, we perform geochemical modeling to show how the distribution of phosphate species found in ice grains from Enceladus's plume provides a very straightforward constraint on the pH of the host solution. The ratio of HPO$_4$/PO$_4$ species serves as a pH indicator. We find evidence of moderately alkaline water (pH 10.1-11.6)--significantly more alkaline than current estimates (~8-9) of the pH of Enceladus's ocean. Nevertheless, the pH range from phosphates is consistent with the CO$_2$/H$_2$O ratio measured in the plume if CO$_2$ exsolves from ocean water according to its equilibrium solubility. A simple energy balance can be used to quantify volatile fractionation during gas transport inside Enceladus's tiger stripes; we deduce that ~83% of water vapor is removed as ice during transport between the liquid-vapor interface and where gases exit the subsurface. We also explore how CO$_2$ degassing may lead to an increase in the apparent pH of ocean water. We generate maps of allowed combinations of pH and dissolved inorganic carbon concentration of the source water for a wide range of scenarios. Our preferred interpretation, constrained by the observed heat flux, implies minimal CO$_2$ degassing from ocean water. Hence, the pH recorded by phosphates should closely approximate that of the ocean; our best estimate is pH ~10.6. Such a high pH seems to reflect a major role of silicates enriched in Na, Mg, or Fe(II) interacting extensively with ocean water. Silica nanoparticles would not form or would subsequently dissolve if the pH is too high (>10.5). The outgassing model presented here provides a new path to quantify the dissolved concentrations of volatile species.