Entrainment and Dynamics of Ocean‐Derived Impurities Within Europa's Ice Shell

Abstract

Compositional heterogeneities within Europa's ice shell likely impact the dynamics and habitability of the ice and subsurface ocean, but the total inventory and distribution of impurities within the shell are unknown. In sea ice on Earth, the thermochemical environment at the ice-ocean interface governs impurity entrainment into the ice. Here, we simulate Europa's ice-ocean interface and bound the impurity load (1.053-14.72 g/kg [parts per thousand weight percent, or ppt] bulk ice shell salinity) and bulk salinity profile of the ice shell. We derive constitutive equations that predict ice composition as a function of the ice shell thermal gradient and ocean composition. We show that evolving solidification rates of the ocean and hydrologic features within the shell produce compositional variations (ice bulk salinities of 5-50% of the ocean salinity) that can affect the material properties of the ice. As the shell thickens, less salt is entrained at the ice-ocean interface, which implies Europa's ice shell is compositionally homogeneous below~1 km. Conversely, the solidification of water filled fractures or lenses introduces substantial compositional variations within the ice shell, creating gradients in mechanical and thermal properties within the ice shell that could help initiate and sustain geological activity. Our results suggest that ocean materials entrained within Europa's ice shell affect the formation of geologic terrain and that these structures could be confirmed by planned spacecraft observations. Plain Language Summary Europa, the second innermost moon of Jupiter, likely houses an interior ocean that could provide a habitat for life. This ocean resides beneath a 10-to >30-km-thick ice shell which could act as a barrier or conveyor for ocean-surface material transport that could render the ocean chemistry either hospitable or unfavorable for life. Additionally, material impurities in the ice shell will alter its physical properties and thus affect the global dynamics of the moon's icy exterior. That said, few of the interior properties of the ice shell or ocean have been directly measured. On Earth, the composition of ocean-derived ice is governed by the chemistry of the parent liquid and the rate at which it forms. Here, we extend models of sea ice to accommodate the Europa ice-ocean environment and produce physically realistic predictions of Europa's ice shell composition and the evolution of water bodies (fractures and lenses) within the shell. Our results show that the thermal gradient of the ice and the liquid composition affect the formation and evolution of geologic features in ways that could be detectable by future spacecraft (e.g., by ice penetrating radar measurements made by Europa Clipper).

The Salty Secrets of Icy Ocean Worlds

“…The study by Buffo et al (2020), one of the first studies seeking to quantify the effects of salinity, considers how specific ocean compositions and concentrations of salt may have been initially entrained into Europa's ice as it formed. By treating the different timescales of formation associated with different thicknesses of ice, the authors are able to explore how different thermal gradients affect the retention of salts in the ice.…”

“…While the work of Buffo et al (2020) provides a needed advancement in terms of adding realistic aqueous chemistry to transport in the ice, it also underscores questions of scaling between Earth's thin ice sheets and the km-scale lithospheres of ocean worlds. The authors considered a top-down freezing scenario, in which initially very thin ice freezes to its present thickness.…”

“…If Europa's ice froze rapidly in a manner similar to the formation of sea ice, regions near the top of the ice that formed first and quickest are likely to have held on to substantial amounts of salt, between 5% and 50% of the salinity of the original fluids. Buffo et al (2020) estimate the bulk amounts of retained salts by considering freezing out of various concentrations of ocean that have been modeled over the years. Their upper-limit concentration for the ocean thus sets the salinity of the ice at 14 ppt by mass, nearly half the 34 ppt concentration of Earth's ocean, and about three times the nominal bulk salinity of multiyear sea ice (e.g., Weeks & Ackley, 1986).…”

“…The work of Buffo et al (2020) provides testable hypotheses, as the presence of salts can affect the dielectric properties of the ice (Pettinelli et al, 2015), which could make the ice penetrating radar (Blankenship et al, 2009) on Europa Clipper susceptible to compositional variations. Due to such effects, regions of high salinity or perched water lenses could become observable from their influence on the reflection and attenuation properties of the ice under the right conditions.…”

“…Due to such effects, regions of high salinity or perched water lenses could become observable from their influence on the reflection and attenuation properties of the ice under the right conditions. Overall, a higher bulk salinity in the ice may cause enhanced attenuation, which would impede efforts to sound to the depth of the ice-ocean interface, while the localized pocket described by Buffo et al (2020) might instead be discerned as discrete features in the radargram. The detection of briny regions within Europa's ice using radar would be a boon to astrobiologists looking for potentially habitable regions in Jupiter's ocean moon.…”