Enceladus as the Most Habitable World We Know of Beyond Earth

Saturn's moon Enceladus essentially has it all when it comes to the requirements of life. Enceladus experiences strong tidal heating as a result of its 2:1 orbital resonance with Dione. This energy source maintains a global, salty water ocean beneath an icy shell. At least some of the tidal energy is converted into chemical energy via hydrothermal activity. The most important consequence of hydrothermal activity for habitability is the production of molecular hydrogen that drives the methanogenesis reaction to a state of disequilibrium. The abundances of CO2, H2, and CH4 in Enceladus's plume indicate that there is enough chemical energy to support methanogens analogous to those on Earth. In addition to an energy source that can be utilized by life, the plume contains numerous other building blocks of life. These include inorganic carbon in the forms of CO2 and carbonate salts, together with a diverse array of organic compounds spanning a wide range of molecular mass and functional group chemistry, including aromatic as well as O- and N-bearing groups. Nitrogen is also available as ammonia. Very recently, sodium phosphate salts were found in ice grains that are sourced from Enceladus's ocean [1]. Phosphorus is now known to be relatively abundant in Enceladus ocean water, which means that P is not a bio-limiting element at Enceladus. A remaining uncertainty with regards to habitability is the availability of sulfur. Hydrogen sulfide was tentatively detected in the plume, while sulfate salts have not been detected. Sulfur is cosmically abundant, although it is currently unclear how much of Enceladus's S inventory is in the ocean vs. present as metal sulfides in the rocky core. Overall, the ocean of Enceladus appears to be a very promising environment for extant life, and the trend has been that it looks more promising with each major research advancement. [1] Postberg et al., Nature, submitted.