The Influence of Mantle Rheology on the Early Differentiation of Icy Satellites

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Icy materials dominated by H2O but also containing ammonia and methane form the bulk of Neptune and Uranus in addition to the 22 major satellites orbiting the outer planets. These satellites, which are believed to have mainly accreted before the dispersal of the solar nebula, are of particular interest to planetary science as they show a range of diverse geological features including the only extraterrestrial evidence for liquid oceans, active plate tectonics and volcanism. The diverse surface expressions of these satellites reflect differing internal processes active since accretion, which have led, for example, to varying degrees of differentiation of the interiors. A key parameter controlling early differentiation was heat loss from the interior through convection. Only bodies that rapidly established convective regimes would have been able to maintain temperatures low enough to prevent melting and complete differentiation, as appears to be the case on Callisto, for example. In order to explore early dynamic processes, detailed constraints on the viscosity of icy materials must be obtained from laboratory measurements of rheological properties. Currently such data are not available for the range of conditions and compositions likely encompassed by the icy satellites. In this project the mechanical properties of ice and icy compounds at pressures and temperatures compatible with the entire range of conditions within icy satellites will be studied. In addition, structural and elastic data will be extracted for many icy materials including clathrates for which data at high pressures are absent. Stress and strain relations will be used to derive flow laws from experiments performed in the diamond anvil cell. These experiments will employ a novel approach of using single crystal x-ray diffraction to determine lattice strains. By examining single and multiple crystal assemblages, fundamental new insights will be made into local stress perturbations within polycrystalline assemblages. Ices are perfect model materials through which to develop such models, which are vital for interpreting high pressure rheological x-ray measurements on silicate materials. Structural, static and dynamic properties of ices will be integrated into models for the internal composition and viscosity of the large icy satellites of Jupiter and Saturn. These models will be used to examine factors during and subsequent to accretion which lead to some satellites differentiating into silicate cores surrounded by ices, while others remained mainly undifferentiated.