| Geodynamic processes on Ganymede and Callisto, the two largest icy satellites of Jupiter, are modeled, providing insight into the thermal and tectonic histories of these two geophysically similar, yet geologically different, moons. A model for unstable lithospheric extension is applied to Ganymede's grooved terrain, a ubiquitous tectonic form composed of subparallel ridge-and-trough sets. By using updated water-ice ductile parameters and by recognizing that low surface temperatures prevailed during groove formation, the model is shown to be viable (especially if local extension exceeds 10%), predicting sufficiently strong instabilities over proper wavelengths, although requiring large near-surface thermal gradients (>10 K km−1) and strain rates (generally >10−15 s−1). The model, however, ignores elasticity, and instability behavior under viscoelasticity is next shown to be different from that under a viscous rheology, lacking constant growth rates and the presence of a dominant wavelength. While such studies provide information on Ganymede during groove formation, study of relaxed impact craters provides information on thermal state for both moons and throughout their lifetimes. Topography creates stresses that temperature-sensitive ductile creep works to relax. Thus, study of crater populations on Ganymede and Callisto, which display a continuum of relaxation states, can be used to probe thermal evolution. Previous analyses had difficulty reconciling rheological parameters for ice with apparent multi-Gyr ages of some terrains. Here, an elastoviscoplastic model for topographic relaxation is developed, wherein under high heat flow, an elastic flexural effect can develop that leads to greater amounts of degradation and to plastic (i.e., brittle) failure at the surface, particularly on the uplifting crater floor. Under reasonable input parameters, the full range of relaxation states can be reproduced. In order to effectively degrade craters in polar regions, the presence of an insulating regolith or a thick frost deposit, to modulate surface temperatures, is indicated. Plastic strains are less than 1%; it is uncertain whether such magnitudes would be observable in available imagery. |
