Evolution of planetary lithospheres: Evidence from multiringed structures on Ganymede and Callisto

Large impact structures on Ganymede and Callisto are characterized by one or more concentric rings or scarps. Their formation is probably due to the collapse of the transient crater when the excavation depth is comparable to the thickness of the planetary lithosphere. The number, spacing, and morphology of the rings is a function of this thickness, the strength of the lithosphere, and crater diameter. When the lithosphere is thin and weak, the collapse is regulated by flow induced in the asthenosphere. The lithosphere fragments in a multiply concentric pattern (e.g., Valhalla, Asgard, Galileo Regio, and a newly discovered ring system on Callisto). The thickness and viscosity of a planetary lithosphere increases with time as the mantle cools. A thicker lithosphere leads to the formation of one (or very few) irregular normal faults concentric to the crater (e.g., Gilgamesh). A gravity wave or tsunami induced by impact into a liquid mantle would result in both concentric and radial extension features. Since these are not observed, this process cannot be responsible for the generation of the rings around the basins and basin palimpsests on Ganymede and Callisto. Subtle differences in thin lithosphere ring morphology between Ganymede and Callisto reflect (at least) the varying ice/silicate ratios, subsequent tectonic histories, and erosional mechanisms of the two bodies. The appearance of Galileo Regio and portions of Valhalla is best explained by ring graben, and though the Valhalla system is older, the lithosphere was 1.5 to 2.0 times as thick at the time of formation. Subsequent tectonic activity destroyed most of the basin-ring structure on Ganymede. The present lithosphere thickness is too great to permit development of any rings.