Porosity and Salt Content Determine if Subduction Can Occur in Europa's Ice Shell

Abstract

Motivated by recent evidence for subduction in Europa's ice shell, we explore the geophysical feasibility of this process. Here we construct a simple model to track the evolution of porosity and temperature within a slab that is forced to subduct. We also vary the initial salt content in Europa's ice shell and determine the buoyancy of our simulated subducting slab. We find that porosity and salt content play a dominant role in determining whether the slab is nonbuoyant and subduction in Europa's ice shell is actually possible. Generally, we find that initially low porosities and high salt contents within the conductive lid are more conducive to subduction. If salt contents are laterally homogenous, and Europa has a reasonable surface porosity of ϕ0 = 0.1, the conductive portion of Europa's shell must have salt contents exceeding ~22% for subduction to occur. However, if salt contents are laterally heterogeneous, with salt contents varying by a few percent, subduction may occur for a surface porosity of ϕ0 = 0.1 and overall salt contents of ~5%. Thus, we argue that under plausible conditions, subduction in Europa's ice shell is possible. Moreover, assuming that subduction is actively occurring or has occurred in Europa's recent past provides important constraints on the structure and composition of the ice shell.

Commentary: The Feasibility of Subduction and Implications for Plate Tectonics on Jupiter's Moon Europa

“…This aspect of the subduction process is nonetheless critical to consider, and the results of the work provide a promising confirmation that the process of progressive subduction is plausible. Ultimately, more rigorous 2‐D or 3‐D viscoelastic and convective models will be needed to confirm these initial results, as Johnson et al () concur. For now, however, the combination of observational and computational models provides strong evidence in support of ice shell subduction on Europa.…”

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A new modeling-based study by Johnson et al (2017, https://doi.org/10.1002 lends support to the hypothesis that portions of Europa's surface may have been removed by the process of subduction, as suggested by Kattenhorn and Prockter (2014, https://doi.org/10.1038/NGEO2245). Using a simple 1-D model that tracks the thermal and density structure of a descending ice plate, Johnson et al show that ice plates with 10% porosity and overall salt contents of~5%, which differ in salt content by~2.5% from the surrounding reference ice shell, are nonbuoyant and thus likely to sink through the underlying, convecting portion of the ice shell.

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“…Nonetheless, assuming these conditions exist on Europa, these models provide a physical explanation for ice‐plate subduction that supports observation‐based inferences of surface removal by subduction, as described by Kattenhorn and Prockter (). The work of Johnson et al () thus provides an insightful test of the likelihood that ice plates are able to sink as subduction proceeds. Future work is nonetheless needed, both observational and numerical, in order to explore the global significance of plate motions on Europa, locations of other possible sites of surface area removal by subduction, and the physical plausibility of the process in a fully integrated 2‐D/3‐D viscoelastic model in which subduction is caused rather than being prescribed.…”

“…The results of the Johnson et al (2017) models clearly present the relative contributions of the thermal anomaly and porosity structure to the overall plate buoyancy. For example, if the initial porosity of the ice exceeds 1%, the porosity characteristics dominate and result in buoyant plates that are unlikely to subduct (although this pattern reverses once the top of the subducting plate reaches a depth of~3 km).…”

“…Europa's surface is heterogeneously mantled by a variety of salts (e.g., Fischer et al, ; McCord et al, ) that may have been brought up to the surface from the interior by convective processes or perhaps deposited on the surface from an external origin, such as adjacent moon Io. The Johnson et al () model results show that these salts become critical to the feasibility of subduction of ice plates on Europa. In response to the differences in salt content between the sinking slab and the underlying convecting ice and the spatial variability in salt content within the sinking slab itself, subducting ice plates may be nonbuoyant from the initiation of subduction (i.e., the plates are consistently likely to sink).…”