GANYMEDE AND CALLISTO - COMPLEX CRATER FORMATION AND PLANETARY CRUSTS

New depth/diameter (d/D), rim, and peak height and width measurements of fresh craters on Ganymede and Callisto are utilized to explore not only crater mechanics on icy satellites but intersatellite crater scaling and crustal properties and composition. Significant systematic differences in crater morphology on icy and rocky planets are confirmed. Simple-to-complex transition diameters on icy satellites, including Ganymede and Callisto, are much lower than those on rocky planets. Also, complex crater depths and rim heights on Ganymede and Callisto are inherently 60-70% shallower than lunar complex craters, despite similar surface gravity. Hence viscous relaxation is not as important as is generally assumed. Central peaks on most icy satellites are 1-2 km higher and considerably wider with respect to diameter than for similar-sized lunar craters. Terracing and rim slumping is also rare in these craters. Central peaks in craters in Ganymede (and Callisto) larger than approximately 15 km, however, are relatively small in both width and height, however, correlating with the occurrence of rim slumping. Thus central peak dimensions are an indirect indicator of the degree of rim slumping. Transition diameters for the occurrence of central peaks and rim slumps and for d/D curve inflections for icy satellites scale approximately with the inverse of surface gravity, except possibly at very low g. The transition diameters for central peaks and for rim slumping differ, however, by 5-15 km on Ganymede and Callisto but by 50 or more km on the smaller icy satellites. These large differences suggest that d/D curves on icy satellites are comprised of three segments of decreasing slope: one for smaller simple craters, a second for central peak craters, and a third for larger true complex craters with both central peaks and rim larger true complex craters with both central peaks and rim slumping. The shallow slopes of d/D curves on Ganymede and Callisto resemble that of the Moon, consistent with the occurrence of both central peaks and rim failure. The high slopes of d/D curves on the middle-sized icy satellites indicate that they are middle segments, consistent with the dominance of central peaks in those craters. Complex crater depths on icy satellites in general scale with the inverse of surface gravity. The depths of all the observed large basin-scale craters on these small satellites, when scaled to Ganymede gravity, are very similar to those of complex craters on Ganymede (and Callisto). These large craters, including Odysseus (425-km diameter) on Tethys, are probably true complex craters that have not undergone significant viscous relaxation. Ithaca Chasma on Tethys probably formed during the prompt collapse of the transient Odysseus crater. Crater morphology is clearly controlled by both gravity and by large variations in material properties (i.e., composition) and should be useful in constraining bulk crustal composition (i.e., ice-rock ratios). The similarity of various morphological transition diameters and complex crater depths on Ganymede and Callisto, two geophysically very similar but geologically divergent large icy satellites, indicates that the crusts of both bodies are dominated by water ice with only a "minor" rocky component.