期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：39
DOI：10.1073/pnas.2201388119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
Enceladus is one of the prime targets in the search for life in the solar system. Observations made by the Cassini spacecraft show that this small moon of Saturn has an ice-covered water ocean that erupts into space, forming a plume that contains almost all of the basic requirements of terrestrial life. However, the bioessential element phosphorus has yet to be found. Here, we perform geochemical modeling, constrained by Cassini data, to predict how much phosphorus could be present in the Enceladus ocean. These models suggest that Enceladus’s ocean should be relatively rich in dissolved phosphorus. This means that there can now be greater confidence that the ocean of Enceladus is habitable.
Saturn’s moon Enceladus has a potentially habitable subsurface water ocean that contains 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 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., HPO
4
2−), and total dissolved inorganic P could reach 10
−7 to 10
−2 mol/kg H
2O, generally increasing with lower pH and higher dissolved CO
2, but also depending upon dissolved ammonia and silica. Levels are much higher than <10
−10 mol/kg H
2O from previous estimates and close to or higher than ∼10
−6 mol/kg H
2O 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 H
2O within 10
5 y, much less than the likely age of Enceladus’s ocean (10
8 to 10
9 y). These results provide further evidence of habitable ocean conditions and show that any oceanic life would not be inhibited by low P availability.
