Presentation #200.01 in the session “Plenary Panel: Active Outer Solar System Satellites”.
The plume of Enceladus provides unique access to fresh material from a habitable, subsurface ocean. Cassini, although not specifically designed to be a seafaring mission, was able to take advantage of the plume to conduct the best characterization to date of an extraterrestrial ocean. Within the Enceladus plume Cassini found evidence for a global, subsurface ocean rich in salts and organics, with water-rock interactions at the seafloor. In particular, a subset of plume particles (~4%) were discovered to contain complex organic molecules with masses greater than 200 u. These organic molecules are found in relatively high abundance (>1% by mass) in these ice particles, and analysis of particles sampled at different velocities indicates that these organics are probably fragments of even larger organic molecules. The Enceladus ocean appears to support a range of organic chemistry, including high-mass compounds diverse in CHON, that could serve as building blocks for, or be the byproducts of, life. Life as we know it requires liquid water and energy as either light or redox disequilibrium. While light energy is not available in the subsurface oceans of Enceladus or any of the ocean worlds, Enceladus’ plume composition is consistent with hydrothermal activity that could sustain redox disequilibrium over long durations. The flux of hydrothermally-sourced H2 and other reduced species into a CO2-rich ocean appears to provide sufficient redox chemical disequilibrium to support microbial metabolism. Alkaline hydrothermal systems on Earth such as Lost City in the Mid-Atlantic Ocean are rich in H2, and may represent the closest analogue to fluids associated with serpentinization at the seafloor of Enceladus. These hydrothermal sites on Earth host teeming ecosystems driven by chemosynthetic organisms. The ocean of Enceladus meets the criteria for habitability, in that it contains liquid water, conditions favorable for the assembly of complex organic molecules, and energy to sustain metabolism. The next step is to investigate whether it is inhabited.