Grain size-dependent viscosity and the thermal evolution of icy satellites

Emma Rainey, California Institute of Technology

Icy satellites are outer solar system objects that are about half rock, half ice by mass. Icy satellites of note include Ganymede, Callisto, and Europa (moons of Jupiter), Titan (moon of Saturn), Triton (moon of Neptune), and large Kuiper Belt objects (Pluto, Quaoar). For large icy objects (greater than about 700 km radius), solid state convection is the dominant method of heat transport in the outer ice layer, so the viscosity of ice is a very important parameter for any model of icy satellite interiors. 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.

Grain size is a dynamic variable in icy satellite interiors. To capture the effects of grain-size dependent viscosity, we use a model for equilibrium grain size that results from a balance between grain growth and grain reduction by dynamic recrystallization. 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 by modifying 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