Modeling icy satellite interiors: effects of grain size-dependent viscosity

Emma Rainey, California Institute of Technology

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With recent Galileo data indicating that Jupiter's moon Europa may have a layer of liquid water beneath its outer ice shell, there is a renewed interest in modeling the interiors of icy satellites. For large enough bodies, solid state convection is the dominant method of heat transport in the outer ice layer. Laboratory experiments on the rheology of ice and field data from ice cores have shown that at low stresses appropriate to planetary conditions, the viscosity of ice is strongly temperature-dependent, grain size-dependent, and weakly stress-dependent (non-Newtonian). The effects of strongly temperature-dependent and stress-dependent viscosity on the thermal evolution of planets have been explored by those studying terrestrial mantle convection. However, the issue of grain size remains largely unexplored, and has only recently begun to be considered for icy satellites.

To capture the effects of grain-size dependent viscosity, I consider an equilibrium grain size that results from a balance between grain growth and grain reduction by dynamic recrystallization, applicable to icy satellites. The resulting expression for mean grain size as a function of stress and temperature can be used in the ice flow law to obtain a viscosity with the same functional form as non-Newtonian temperature-dependent viscosity, with an effective stress exponent and effective activation energy. Therefore, the effects of grain size on the thermal state of icy satellites can be modeled using standard numerical codes for mantle convection with non-Newtonian rheology. Preliminary results will be presented.

Abstract Author(s): Emma S.G. Rainey<br />David J. Stevenson