Geometry of Freezing Impacts Ice Composition: Implications for Icy Satellites

Abstract

Non‐ice impurities within the ice shells of ocean worlds (e.g., Europa, Enceladus, Titan, Ganymede) are believed to play a fundamental role in their geophysics and habitability and may become a surface expression of subsurface ocean properties. Heterogeneous entrainment and distribution of impurities within planetary ice shells have been proposed as mechanisms that can drive ice shell overturns, generate diverse geological features, and facilitate ocean‐surface material transport critical for maintaining a habitable subsurface ocean. However, current models of ice shell composition suggest that impurity rejection at the ice‐ocean interface of thick contemporary ice shells will be exceptionally efficient, resulting in relatively pure, homogeneous ice. As such, additional mechanisms capable of facilitating enhanced and heterogeneous impurity entrainment are needed to reconcile the observed physicochemical diversity of planetary ice shells. Here we investigate the potential for hydrologic features within planetary ice shells (sills and basal fractures), and the unique freezing geometries they promote, to provide such a mechanism. By simulating the two‐dimensional thermal and physicochemical evolution of these hydrological features as they solidify, we demonstrate that bottom‐up solidification at sill floors and horizontal solidification at fracture walls generate distinct ice compositions and provide mechanisms for both enhanced and heterogeneous impurity entrainment. We compare our results with magmatic and metallurgic analogs that exhibit similar micro‐ and macroscale chemical zonation patterns during solidification. Our results suggest variations in ice‐ocean/brine interface geometry could play a fundamental role in introducing compositional heterogeneities into planetary ice shells and cryoconcentrating impurities in (re)frozen hydrologic features.

Salt Distribution from Freezing Intrusions in Ice Shells on Ocean Worlds: Application to Europa

“…Unlike in a chamber, the salty water drains into the ocean below and the fluid is maintained at the ocean composition and salinity. Intermediate cases have been investigated in more physical detail by Buffo et al (2020), and for NaCl-or MgSO 4 -bearing fluid by Buffo et al (2023). In the latter study, partial brine entrapment and rejection leads to finer-scale, complex compositional structure that is not captured by our analysis, with chemical zonation patterns reminiscent of metallurgic analogs (Buffo et al 2023).…”

“…In our approach, we make the simplifying assumption that the ice-brine interface remains sharp, as opposed to mushy (Vance et al 2019;Buffo et al 2020Buffo et al , 2021aBuffo et al , 2021bBuffo et al , 2022Buffo et al , 2023. This notably neglects the typical mechanism for the entrainment of salt in ice, which involves the trapping of brine pockets at the ice-brine interface.…”

“…The combination of this process and gravity drainage (Notz & Worster 2009;Vance et al 2019;Buffo et al 2021aBuffo et al , 2021bHesse et al 2022;Wolfenbarger et al 2022a) is not considered in the present model, but is expected to result in a general downward migration of brine, either as a partial loss from the upper chamber wall back into the central pocket of remaining liquid (e.g., Buffo et al 2020), or within the chamber sidewalls. Indeed, two-dimensional freezing simulations of sills with upper and lower ice walls indicate downward brine migration out of the upper wall, but no brine migration from the lower wall (Buffo et al 2023).…”

“…The encounter between ascending warm ice and perched salt-bearing ice has been proposed (Schmidt et al 2011) as a cause for generating perched melt lenses to explain surface chaos features on Europa. The formation and evolution of Europa's ice shell, basal fractures, and shallower intrusions of liquid have been simulated using a model informed by observations of saline ice formation and composition on Earth (Buffo et al 2020(Buffo et al , 2023, and assuming a starting ocean composition from Zolotov & Shock (2001). These simulations have suggested that the solidification of introduced fluids could result in compositional variations within the ice shell substantial enough to affect its thermophysical properties.…”