The Carbonate Geochemistry of Enceladus' Ocean

• Published in: Geophysical research letters Vol. 47; no. 3
• Main Authors: Glein, Christopher R., Waite, J. Hunter
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.02.2020
• Subjects: Carbon; Carbon dioxide; Carbon dioxide concentration; Carbonate minerals; Carbonates; Carbonation; Complexity; Composition; Computational fluid dynamics; Datasets; Echoes; Enceladus; Energy resources; Energy sources; extraterrestrial oceanography; Fluids; Gases; Geochemistry; Habitability; Heterogeneous structure; Indoor environments; Mass spectrometry; Mass spectroscopy; Minerals; Ocean floor; ocean world; Oceans; Rock; Rocks; Saturn satellites; Sea spray; Silica; Silicon dioxide; Survival
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL085885

The plume composition at Enceladus contains clues about conditions and processes in the interior. We present new geochemical interpretations of Cassini mass spectrometry data from the plume gas and salt‐rich ice grains. It is found that self‐consistency between the data sets can be achieved with a derived range of 10−4.6 to 10−3.2 for the activity of CO2 in Enceladus' ocean. This range is compatible with long‐term buffering by reduced or oxidized seafloor rocks containing quartz, talc, and carbonate minerals in the MgO–FeO–SiO2–CO2–H2O system. Reaction path modeling shows that these types of rocks can be produced from accreted CO2‐rich fluids reacting with hydrous chondritic rocks over an intermediate regime of carbonation. These results, together with previous findings of silica and H2 at Enceladus, support the hypothesis of a heterogeneous structure for the rocky core (carbonated upper layer, serpentinized interior), which provides a geochemical gradient for habitability. Plain Language Summary Enceladus, an ocean‐harboring moon of Saturn, erupts a plume that contains gases and frozen sea spray into space. By understanding the composition of the plume, we can learn about what the ocean is like, how it got to be this way, and whether it provides environments where life as we know it could survive. This study presents a new perspective for analyzing the plume composition to estimate the concentration of dissolved carbon dioxide in the ocean. We find that the derived range based on two different data sets is intriguingly similar to what would be expected from the dissolution and formation of certain mixtures of silicon‐ and carbon‐bearing minerals at the seafloor. The deduced combination of minerals may be indicative of a fundamental process that has sequestered a large amount of Enceladus' initial inventory of carbon dioxide into the rocky core. This inference echoes an emerging vision of a complex interior that hosts geochemically diverse environments. The dynamic interface of such complexity is where energy sources for possible life may arise. Key Points New estimates of the thermodynamic activity of CO2 in Enceladus' ocean based on the carbonate chemistry of the plume Formation of CO2‐buffering assemblages of quartz‐talc‐carbonate from partial carbonation of chondritic compositions Distinct sources of observed CO2, silica, and H2 imply mineralogically and thermally diverse environments in the rocky core