Enceladus: Evidence and Unsolved Questions for an Ice-Covered Habitable World
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The icy midsized satellite of Saturn—Enceladus—has become the central to astrobiology since the finding of its dramatic ongoing geological activity. The water-rich plumes erupting from the warm fractures on the icy crust near the South Pole of Enceladus originate from its global subsurface ocean that interacts with the rocky core. In situ measurements of the plume by the Cassini spacecraft showed that the ocean contains dissolved gas species, such as CO2, NH3, CH4, and H2, which can provide disequilibrium redox energy to support methanogenic life. The salt composition of the plume indicates an alkaline pH of the ocean (pH ~9 to 11). The plume also contains significant amounts of organic matter, including high-molecular-weight organic compounds, although its origin remains unclear. Ongoing hydrothermal activity at temperatures greater than 90 °C is highly likely to exist on the seafloor or within the rocky core, which could play a role in sustaining the chemical disequilibrium within the ocean. These observations suggest that Enceladus is a planetary body thus far that currently meets the fundamental requirements for habitability and life—liquid water, organic matter, and bioavailable energy—beyond Earth.
KeywordsIcy satellite Geochemistry Habitability
This work was supported by MEXT KAKENHI Grant Number JP 17H0655, 17H06456, and 17H06457.
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