The subsurface ocean of Enceladus: A habitable place in our solar system ?

Main Colloquium
Prof. Frank Postberg
Institute of Geological Sciences, Freie Universität Berlin

Saturn's icy moon Enceladus harbours a global ocean, which lies under an ice crust of just a few kilometres thickness and above a rocky core. Through warm cracks in the crust a cryo-volcanic plume ejects ice grains and vapour into space providing access to materials originating from the ocean. The ocean is 30-55 km deep and provides an environment of mild salinity and alkaline pH. Hydrothermal activity is suspected to be occurring at the bottom of the ocean and also deep inside the water-percolated porous core. The energy is delivered by tidal dissipation. Two mass spectrometers aboard the Cassini spacecraft, the Cosmic Dust Analyzer (CDA) and the Ion and Neutral Gas Spectrometer (INMS) frequently carried out compositional in situ measurements of plume material emerging from the subsurface of Enceladus. Our latest results now show that, in addition to volatile organic compounds, some emitted ice grains contain concentrated macromolecular organic material with molecular masses clearly above 200u. The mass spectra of the two instruments provide key constraints on the macromolecular structure that contains both aromatic and aliphatic constituents as well as oxygen bearing and likely nitrogen bearing functional groups. The finding is suggestive of thin organic-rich film on top of the oceanic water surface. We suggest that the detected organic compounds mostly originate from Enceladus' hydrothermally active rocky core. Thermal ocean convection together with bubbles of volatile gases, transports these and other materials from the moon's core up to the ocean surface. There, organic nucleation cores - generated by bubble bursting and subsequently coated with ice from vapor freezing - are ejected into space. This nucleation shows similarities to the formation of ice clouds from organic sea spray on Earth and allows probing of Enceladus' organic inventory in drastically enhanced concentrations.