Constraining the outgassing history of Enceladus’s ocean with noble gases

Enceladus is a compelling place to search for life within our own Solar System as this icy moon harbors a global liquid water ocean beneath its icy crust [1] as well other basic ingredients for habitability. The detection of nearly pure nanometer-sized silica [2] and the excess amount of native molecular hydrogen in the plume [3] both indicate ongoing hydrothermal activities. The ocean also contains organics, including the macromolecular ones [4]. Despite what Cassini has revealed us about Enceladus as an intriguing ocean world, questions regarding the formation and evolution of Enceladus are still unknown. In particular, the sustainability of the ocean and its variability with time are still under debate, with estimates ranging from a few ten Myr to Gyr [5,6,7]. Dating chronology in the outer solar system is challenging, however novel dating methods are crucial to infer the evolution of icy ocean worlds [8]. Noble gases are chemically inert, and therefore could be used as a robust tracer to understand geological processes. Here, we present an outgassing model to constrain the outgassing duration of the plume from the evolution of noble gases with potential applications to other icy satellites. Our kinetic model considers the fractionation of volatiles at the ocean-vapor interface (i.e., bubble-induced transfer, turbulence, boiling under vacuum) as well as water freezing during the vapor ascending within the crack. As a consequence of plume outgassing, significant amounts of noble gases may have been lost over time if the ocean is old (~ Gyr). Another prediction is the enrichment of heavy isotopes, both are testable in a future mission equipped with high-resolution mass spectrometers used in flythroughs of the plume [9]. Future work may also need to include the role of clathrates trapping in replenishing the noble gases from the ice shell to the ocean. References: [1] Thomas et al., 2016, [2] Hsu et al., 2015, [3] Waite et al., 2017, [4] Postberg et al., 2018, [5] Daval et al., 2021, [6] Cuk et al., 2016, [7] Lainey et al., 2020, [8] Schenk et al., 2021, [9] Lunine, 2017