Crater formation and modification on the icy satellites of Uranus and Saturn: Depth/diameter and central peak occurrence

Abstract

Depth/diameters (d/D) for fresh craters on the intermediate‐sized icy satellites of Uranus and Saturn have been determined using photoclinometry and shadow lengths, and are compared with similar measurements on the terrestrial planets. Simple bowl‐shaped craters on icy satellites, including those on Miranda for which the highest resolution data are available, are systematically 20–40% shallower than on the terrestrial planets. This pronounced difference between crater depths on icy and rocky surfaces indicates that differences in impact velocity or surface gravity are not as important as the differences in mechanical properties between ice and “rock” in controlling simple crater morphology. Experimental impact studies indicate that differences in material properties such as angle of internal friction can control crater depth. Alternatively, breccia lenses may be thicker in icy satellite simple craters. Complex craters on the icy satellites become significantly deeper with increasing crater diameters, unlike complex craters on terrestrial planets, which are nearly constant in depth. Central peaks, and hence floor rebound, are also volumetrically and morphologically more prominent than wall slumping (rim collapse) on the icy satellites. The magnitude of viscous relaxation of very large craters (e.g., Herschel and Odysseus) on the icy satellites can be estimated from extrapolation of the d/D fits at smaller crater diameters. The transition diameter from simple to complex morphology is inversely correlated with gravity, as it is on the terrestrial planets, but at significantly smaller diameters than would be expected from a simple extrapolation of the terrestrial trend. Thus crater modification, especially floor rebound, is easier to initiate on icy satellites. Estimated effective cohesions for shocked icy material are lower by a factor of 10–20 relative to rock, although estimated effective viscosities are similar. The distinct, systematic differences between crater morphology on icy and rocky worlds indicate that gross material property differences between rock and ices play a key role in crater formation and modification, but gravity is still the primary driving force.