Abundant phosphorus for life in the Enceladus ocean

Abstract:

Saturn’s moon Enceladus has a potentially habitable subsurface water ocean that contains many of the canonical building blocks of life (organic and inorganic carbon, ammonia, possibly hydrogen sulfide) and chemical energy (disequilibria for methanogenesis). However, its habitability could be strongly affected by the availability of phosphorus (P). Here, we perform thermodynamic and kinetic modelling that simulates P geochemistry based on recent insights into the geochemistry of the ocean-seafloor system on Enceladus. We find that P should predominantly exist as orthophosphate (e.g., HPO₄^2-), and total dissolved P could reach levels (10^-2 to 10^-7 moles/kg H₂O) much higher than or close to those in modern Earth seawater. The high P concentration is primarily ascribed to a high (bi)carbonate concentration, which drives the concentrations of multivalent cations down via carbonate mineral formation, allowing phosphate to accumulate. Kinetic modelling of phosphate mineral dissolution suggests that geologically rapid release of P from seafloor weathering of a chondritic core could supply mmolal total dissolved P over a timescale of thousands of years or less, much less than the likely age of Enceladus (> 100 Ma to several Ga). These results provide further evidence of habitable conditions in the Enceladus ocean, and if life exists there, its productivity should not be limited by the availability of P.