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Convection in Icy Satellites with Self-Consistent Grain Size

Introduction: Ice grain size controls the geodynamic evolution of the ice I shells of large to mid-sized icy satellites. Grain size controls the viscosity of ice [1,2], and thus, the heat flux from the outer ice I shell by controlling the likelihood and efficiency of convection. In addition, the rheology of ice controls the depth to the brittle/ductile transition, which is a key control on endogenic resurfacing. In a satellite that experiences rapid tidal flexing (i.e., Europa), the ice grain size additionally controls the spatial distribution of tidal dissipation. Temperature, stress, and strain rate conditions in many terrestrial ice sheets (e.g., the West Antarctic and Greenland Ice Sheets) are similar to conditions expected within the outer ice I shells of large to midsized icy satellites [3,4]. Therefore, ice grain size measurements from terrestrial ice sheets may provide information about grain sizes expected within the satellites. Grain size within terrestrial ice sheets is controlled by both the concentrations of non-waterice impurities, and by dynamic recrystallization, where the average ice grain size is modified as strain accumulates. We use measurements of grain size from the Greenland ice sheet to constrain grain size in the satellites to determine the conditions where convection can occur in the shells, and if convection occurs, what grain sizes are expected in the convecting sub-layer. Dynamic Recrystallization: As ice flows, grains with a high dislocation density are absorbed by relatively dislocation-free grains in a process called migration recrystallization or grain boundary migration. In addition, heterogeneous deformation within ice grains leads to the formation of new grain boundaries in a process called rotation recrystallization. These processes are collectively referred to as dynamic recrystallization. Observations of recrystallized grain size in metals and ice suggest that the grain size (d) is proportional to stress (σ) as d ~ Aσ where m~1 [5]. Models of recrystallized grain size suggest that grain size is related to stress as [6]