Age estimates of Enceladuss ocean and other outer solar system icy ocean worlds inferred from the evolution of noble gases

Many observations have pointed out Enceladus as a compelling place for astrobiology as this icy moon contains most basic ingredients for habitability: a global liquid water ocean beneath its icy crust [1], some indications of ongoing hydrothermal activities [2,3], and the presence of macromolecular organics [4]. An outstanding remaining question is the lifetime of the ocean, 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 essential 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 as evident by their roles in our understanding of Earths evolution [9]. Here, we present an outgassing model to constrain the age of Enceladuss ocean from the evolution of noble gases with potential applications to other icy satellites. First, we calculate the partition coefficient in a wide range of temperature and pressure for different noble gases, which determine their distributions in aqueous and vapor phases respectively. Second, we calculate the residual abundance for different species as the remaining liquid water continuously evaporates to become vapor, which our model predicts closely matches with the existing result for Yellowstone [10]. 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 [11]. 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] Mukhopadhyay & Parai, 2019, [10] Gardner et al., 2010, [11] Lunine, 2017