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Internal Evolution and Current State of Enceladus Accreted from Cometary Material

Presentation #313.02 in the session Enceladus (Poster)

Published onOct 23, 2023
Internal Evolution and Current State of Enceladus Accreted from Cometary Material

Observations of Enceladus’ plume composition have led to the suggestion that the moon formed from cometary material (Waite et al., Nature 2009; Glein and Waite, GRL 2020; Zolotov, Nature 2023). This implies that it accreted a large fraction of organic matter and carbon-bearing ices. For example, results obtained by the Rosetta mission indicate that comet 67P contains about 50 wt.% of organics (Fulle et al. MNRAS 2017).

Carbon-bearing ices may yield bi/carbonate and ammonium ions in solution and can also produce clathrate hydrates, depending on Enceladus’ internal conditions. The apparent discrepancy between the density of Enceladus’ ocean inferred from the Cassini composition measurements (see Glein et al., Enceladus and the Icy Moons of Saturn 2018 for a review) and the salinity predicted for carbon-rich models (e.g., Castillo-Rogez et al., GRL 2022) may point to significant fractionation of the ocean material on its journey to the surface (e.g., Fifer et al., PSJ 2022).

If Enceladus entirely formed from cometary materials, IOM is so pervasive that it replaces porosity in the core and represents an alternative way to explain the core low density. Models of tidal heating produced from hydrothermal circulation in the core are not viable in this case, and new mechanisms need to be introduced in order to produce significant tidal heating inside Enceladus’ core. A promising direction is based on the few experimental studies (e.g., Behura et al., The Leading Edge 2009) showing that oil shales dissipate heat in response to tidal stressing (down to 10-2 Hz) at relatively low temperatures (~150oC). Another feature of IOM is that it might represent a source of volatiles (e.g., CO2, CH4, and H2) and heavier organics from degradation under low-grade heating (e.g., Waite et al., Science 2017; Postberg et al., Nature 2018; Nakano et al., Scientific Reports 2020).

We will present new models of Enceladus that integrate these features for different fractions of accreted cometary material. We encourage future experimental research on measuring these properties over a wide range of conditions relevant to icy moons and dwarf planets since large fractions of accreted OM could be a general characteristics of these bodies (Reynard and Sotin, EPSL 2023).

We will also make predictions about the isotopic ratios of hydrogen, carbon and nitrogen, which can be tested by future missions.

Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration.

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