Reaching Europa’s Surface: Erosion of the Viscous Lid by Compositional Plumes with Implications for Ocean-Surface Material Exchange

Abstract:

The source of the intriguing surface chemistry of the icy moon, Europa has been widely debated. Of primary interest is the process of surface-ice-shell-ocean exchange of trace chemistry. Of particular astrobiological interest is the possible endogenic origin of the surface chemistry. The viscous lid atop the ice-shell poses a physical barrier for such a material exchange to occur. We have performed a computational study of thermochemical convection models to test the weakening of this viscous lid by warmer plumes of lower compositional density rising in the ice-shell. We modeled a two-phase convecting ice-ocean system with a low viscosity proxy fluid approximation for the liquid ocean. On achieving a stable convecting system, the newly frozen ice at the base of the warm ice plumes at the shell-ocean boundary is tracked and mapped as it is advected upwards by the rising plumes. The newly formed ice is prescribed a lower intrinsic density than the ambient ice to mimic the scenario where compositional contrasts in the ice-shell could exist. We then study how the rising compositional plumes incorporated with the low intrinsic density new ice can erode the stagnant lid at the top of the ice-shell. Several values of density contrast have been modeled to study any variability in their extent of erosion of the lid.

The models show that in a convecting ice-ocean system, it is possible for less dense newly formed ice to incorporate into the convecting ice plumes and erode the lid over time. The results suggest that if oceanic trace chemistry were to be incorporated into the newly frozen ice at the ice-ocean interface, it could be possible for it to reach the surface by continual erosion of the viscous ice lid at the top of the shell. This presents a plausible scenario for surface detection of an endogenic chemical signature that could be a potential biosignature of subsurface life in Europa.