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

除了围绕外层行星运行的22颗主要卫星外，海王星和天王星的大部分都是由以水为主、但也含有氨和甲烷的冰状物质构成的。这些卫星被认为主要是在太阳星云分散之前形成的，它们对行星科学特别感兴趣，因为它们显示了一系列不同的地质特征，包括唯一的外星证据，表明存在液态海洋、活跃的板块构造和火山作用。这些卫星的不同表面表达反映了自吸积以来活跃的不同内部过程，例如，这些过程导致了内部不同程度的分化。控制早期分化的关键参数是内部通过对流的热损失。只有快速建立对流机制的天体才能够保持足够低的温度，以防止融化和完全分化，比如木卫四上的情况。为了探索早期的动态过程，必须从实验室流变特性的测量中获得冰材料粘度的详细限制。目前这类数据还无法提供给冰态卫星可能包含的条件和成分范围。在这个项目中，将研究冰和冰化合物在与冰卫星内所有条件相适应的压力和温度下的机械性能。此外，还将提取包括包合物在内的许多冰冻材料的结构和弹性数据，这些材料在高压下没有数据。应力和应变关系将用于从金刚石砧室中进行的实验中得出流动规律。这些实验将采用单晶x射线衍射的新方法来确定晶格应变。通过研究单晶和多晶组合，将对多晶组合中的局部应力扰动产生新的见解。冰是完美的模型材料，通过它可以建立这样的模型，这对于解释硅酸盐材料的高压流变x射线测量至关重要。冰的结构、静态和动态特性将被整合到木星和土星的大型冰卫星的内部组成和粘度模型中。这些模型将用于检查吸积期间和之后的因素，这些因素导致一些卫星分化为被冰包围的硅酸盐核心，而其他卫星主要保持未分化。

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.