Europa's Surface and Shallow Water: Ice Shell Activity and Implications for Habitability

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Chivers, Chase James
Schmidt, Britney
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Beneath the geologically complex ice shell of Europa, Jupiter’s innermost icy satellite, likely lies a vast, saline subsurface ocean that may hold conditions favorable for life. Key to that question is how processes in the ice shell, represented as a myriad of geologic features on the surface, facilitate material transport between the surface and subsurface ocean. The formation of the young, elliptically shaped surface disruptions, lenticulae and chaotic (chaos) terrain that range from < 10 km to > 1000 km diameter, may represent one such process. Recent geologic analyses of the Galileo spacecraft observations suggests that both lenticulae and chaos terrain may form by reservoirs of saline liquid water emplaced as shallow as 1 km below the surface, or the so-called “shallow water” model. Lenticulae may form via the injection and freezing of liquid water sills < 10 km in diameter; Chaos terrain may form via local eutectic melting of the ice shell creating a “melt lens” > 50 km in diameter. In this thesis, I aim to link observations and formation hypotheses to theoretical numerical models that define hypothesis tests to motivate future observations for upcoming flyby mission NASA’s Europa Clipper. To that end, I developed a multiphase, two-dimensional, finite difference model that describes the thermal and chemical evolution of saline, shallow water reservoirs after they are emplaced in Europa’s ice shell. Built on the foundations of terrestrial sea ice formation by applying the microphysical process of mushy layer development, I can track the distribution of salts within the ice shell during and after the solidification of these saline reservoirs to predict both their longevity within the ice shell and how they may be detected by future missions. I show that while the liquid water within injected sills beneath lenticulae are shorter lived than previous estimates < 140,000 years, the interpretation of their geomorphology suggests liquid water is present within the ice shell. Similarly, I show that melt lenses, owed to its origin as localized melt, are much longer-lasting, at least 175,000 years and potentially remain as a quasi-stable brine volume < 300 cubic km for 100,000s of years after. Observations have suggested the contemporary presence of liquid water in the ice shell but had not yet explicitly modeled or tested before. The solidification of injected sills and melt lenses predicts distinct chemical zoning patterns based on its chemistry and environmental factors. In particular, I show that the brines filling injected sills easily reach their eutectic concentration during solidification, regardless of chemistry, and can precipitate up to ~50% of the initial salt content, leaving behind layers of precipitated hydrated salts within the ice shell. Heterogeneous distributions of entrained salts through solidification and precipitated salt layers had also been proposed in the past, but no predictions on their extent or frequency had yet been made. Synthesizing my results, the current-best estimates of shallow water solidification and salt entrainment, I show that a number of investigations may be able to confirm the presence of liquid water in the ice shell through the lens of the Europa Clipper instrument suite. The heterogeneous distribution of salts and the presence of brines within the ice shell has significant implications for geophysical processes in the ice shell and its habitability.
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