Abundant phosphorus expected for possible life in Enceladus’s ocean
Saturn’s moon Enceladus has a potentially habitable subsurface water ocean that contains canonical building blocks of life (organic and inorganic carbon, ammonia, possibly hydro-gen sulfide) and chemical energy (disequilibria for methanogenesis). However, its habit-ability could be strongly affected by the unknown availability of phosphorus (P). Here, we perform thermodynamic and kinetic modeling that simulates P geochemistry based on recent insights into the geochemistry of the ocean–seafloor system on Enceladus. We find that aqueous P should predominantly exist as orthophosphate (e.g., HPO422), and total dissolved inorganic P could reach 1027to 1022mol/kg H2O, generally increasing with lower pH and higher dissolved CO2, but also depending upon dissolved ammonia and silica. Levels are much higher than<10210mol/kg H2Ofrompreviousestimatesand close to or higher than∼1026mol/kg H2O in modern Earth seawater. The high P concentration is primarily ascribed to a high (bi)carbonate concentration, which decreases the concentrations of multivalent cations via carbonate mineral formation, allowing phosphate to accumulate. Kinetic modeling of phosphate mineral dissolution suggests that geologically rapid release of P from seafloor weathering of a chondritic rocky core could supply millimoles of total dissolved P per kilogram of H2Owithin105y, much less than the likely age of Enceladus’s ocean (108to 109y). These results provide further evidence of habitable ocean conditions and show that any oceanic life would not be inhibited by low P availability.