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Sym, Lewis; Ramkissoon, Nisha; Fox-Powell, Mark; Pearson, Victoria and Melwani Daswani, Mohit
(2024).
Abstract
Introduction: Europa is proposed to host a global liquid water ocean that is in contact with a silicate interior (Sotin et al., 2009). Understanding the composition of this ocean and the underlying rock is crucial for evaluating the habitability of Europa. However, the presence of an ice shell impedes direct observation or analysis of the ocean and rock, leaving their compositions largely unknown. Previous modelling work has shown that, if Europa accreted entirely from CI or CM chondritic material, sufficient volatiles could be released during prograde metamorphism to account for the current size of the hydrosphere (Melwani Daswani et al., 2021). However, thermal models predict that temperatures in Europa’s interior would gradually increase over billions of years (e.g. Trinh et al., 2023), where the progressive release of volatiles would change the ocean composition over time. In this study, possible ocean compositions were explored using computer modelling to simulate the thermal evolution of Europa’s interior over its ~4.5 Gyr lifetime and assess the volatiles released from the starting material as it is heated.
Methods: The composition of Murchison (a CM chondrite) was used to represent the silicate material that accreted to form Europa because the CMs formed close to early Jupiter (unlike the CIs; Desch et al., 2018) and contain sufficient water (largely held within hydrated silicates; Howard et al., 2011). A 1-dimensional thermal evolution code was used to model the temperatures achieved within Europa’s interior (Trinh et al., 2023). Temperature-depth profiles were then extracted at two points in time to reflect the formation of the proto-ocean (i.e. ~1600 Myr since the calcium-aluminium-rich inclusions (CAIs)) and the current-day ocean (~4568 Myr since the CAIs). Rcrust (Mayne et al., 2016) and Perple_X (Connolly, 2005) were used to calculate the electrolytic fluid speciation from the starting material when heated to the temperatures predicted by the first temperature-depth profile (Stage 1; 4 – ~1600 Myr) and then the second (Stage 2; ~1600 – ~4568 Myr). Pyrrhotite was extracted from the starting material past the Fe-FeS eutectic temperature to approximate core formation. The volatiles representing the metamorphic contribution to the proto-ocean (i.e. those released in Stage 1) were then equilibrated using CHIM-XPT (Reed et al., 2010). Further volatiles (i.e. those released in Stage 2) were then added to the proto-ocean in CHIM-XPT, predicting the metamorphic contribution to the current-day ocean.
Results and Discussion: Released volatiles for the proto-ocean are predicted to form a ~91.4 km deep layer around Europa. With the addition of the Stage 2 volatiles, the current-day ocean would be ~100.7 km deep. The extraction of pyrrhotite, which occurs after proto-ocean formation, would form a metallic core of ~288.3 km radius by the current day. The current-day ocean depth and core radius predicted here agree with those inferred for current-day Europa based on observations (Trinh et al., 2023). The model predicts that both the proto- and current-day oceans would be rich (>10-2 mol kg-1) in Na+, Cl-, and CO32-, which may explain the recent observations of NaCl and CO2 in geologically-disrupted regions of Europa’s surface (Trumbo et al., 2019; Villanueva et al., 2023). However, large concentrations of NH3 and NH4+ are predicted for both the proto- and current-day oceans, despite the lack of any clear detection of nitrogen species on the surface. A potentially important difference between the proto- and current-day oceans is their CH4 and HS- concentrations, where the current-day ocean has only ~17% and ~23% that of the proto-ocean, respectively. This decrease in CH4 and HS- content could affect the potential for energy generation by methane- and sulfide-oxidising microbes in the current-day ocean.
Conclusion: We find that Europa’s ocean composition would have varied over time as a result of continued prograde metamorphism, and that this would have implications for Europa’s continuous habitability.
Acknowledgements: This project is funded by the Science and Technology Facilities Council. 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.
References:
Connolly J. A. D. (2005) Earth Planet. Sci. Lett., 236, 524–541.
Desch S. J. et al. (2018) ApJS., 238.
Howard K. T. et al. (2011) Geochim. Cosmochim. Acta., 75, 2735–2751.
Mayne M. J. et al. (2016) J. Metamorph. Geol., 34, 663–682.
Melwani Daswani M. et al. (2021) Geophys. Res. Lett., 48.
Reed M. H. et al. (2010) J. Chem. Inf. Model., 53, 1689–1699.
Sotin C. et al. (2009) in Europa, pp. 85–117.
Trinh K. T. et al. (2023) Sci. Adv., 9, eadf3955.
Trumbo S. K. et al. (2019) Sci. Adv., 5, eaaw7123.
Villanueva G. L. et al. (2023) Science., 381, 1305–1308.
Plain Language Summary
Jupiter's fourth largest moon, Europa, is thought to host a global ocean beneath its icy surface. Understanding the composition of this ocean is important for evaluating Europa's potential to support life – that is, its habitability. However, the ice shell prevents any direct observation of the ocean, or its underlying seafloor, leaving the ocean and mantle compositions largely unknown. Here, we have used computer models to simulate how Europa's ocean and mantle compositions may have changed over time as the interior heated. We find that simulating Europa's evolution in this way can produce a model of current-day Europa that is in good agreement with telescopic and spacecraft observations. Further, we postulate how the changing ocean composition could affect the resources available to life in Europa's ocean as time progresses.
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