Craterform and related features on Ganymede and Callisto include bowl-shaped craters, craters with nearly fiat floors, craters with central peaks, craters with central pits, basins, crater palimpsests and penepalimpsests, and giant multiring systems of ridges and furrows. The large majority of all craters larger than 20 km diameter have a central pit. The pits are interpreted as formed by prompt collapse of transient central peaks. Most craters, in all size ranges, are highly flattened as a consequence of topographic relaxation by slow viscous or plastic flow.
Analysis of the global distribution of craters and multiring structures on Callisto reveal that the large multiring structures are concentrated in the leading hemisphere, whereas craters are depleted here. Calculations of model crater retention ages based on a sample of 2000 craters ≥ 30 km in diameter show that the mean age of Callisto's surface is between 4.0 and 4.2 Gy. Variations in the surface ages, derived from different diameter craters, suggests that larger craters are not retained from as early a period in time as were the smaller craters; this is in agreement with the results predicted by viscous relaxation theory where large wavelength features relax at a faster rate than do small wavelength features. Most of the variations in the observed distribution of craters can be explained satisfactorily by the effects due to the formation of multiring structures, and on the viscous relaxation of craters beneath an insulating regolith.
About 1000 topographic profiles of craters on Ganymede and Callisto were obtained by photoclinometry. Fresh craters on Ganymede and Callisto have depth-to-diameter ratios and rim height-to-diameter ratios similar to those of fresh lunar craters, but most craters are much shallower. Small craters have not flattened or relaxed as much as have large craters; comparison of the crater profiles with the results from theoretical of crater relaxation studies in a viscous medium, allows determination of the viscosity at the surfaces of Ganymede and Callisto, and, also, determination of the viscosity gradient with depth. The derived mean surface viscosity for the lithospheres of Ganymede and Callisto is 1.0 ± 0.5 x 1026 poise. For Ganymede, the estimated thermal gradient at ~3.9 Gya was ≥ 8 K/km; the thermal gradient can be modelled as decreasing approximately exponentially with time, with an e-folding time of about 108 years; the estimated present thermal gradient is ≤ 2.0 K/km. For Callisto, the thermal gradient was ≥ 3 K/km at ~4.1 GYA and the decrease in the thermal gradient can be modelled as an exponential dropoff with an e-folding time between about 5 x 107 and 2 x 108 years; the estimated present thermal gradient on Callisto ≤ 1.5 K/km.
High resolution Voyager II images of Enceladus reveal that some regions on its surface are highly cratered; the most heavily cratered surfaces probably date back into a period of heavy bombardment. The forms of many of the craters, on Enceladus, are similar to those of fresh lunar craters, but many of the craters are much shallower in depth, and the floors of some craters are bowed up. Analysis of the forms of the flattened craters on Enceladus suggests that the viscosity at the top of the lithosphere, in the most heavily cratered regions, is between 1024 and 1025 poise. The exact time scale for the collapse of the craters is not known, but probably was between 100 My and 4 Gy. The flattened craters are located in regions in which the heat flow was (or is) higher than in the adjacent terrains. Because the temperature at the top of the lithosphere of Enceladus would be less than, or equal to that of Ganymede and Callisto, if it is covered by a thick regolith, and because the required viscosity, on Enceladus, is one to two orders of magnitude less than for Ganymede and Callisto, it can be concluded that the lithospheric material, on Enceladus, is different from that of Ganymede and Callisto. Enceladus possibly has a mixture of ammonia ice and water ice in the lithosphere, whereas the lithospheres of Ganymede and Callisto are composed primarily of water ice.
New field measurements of elevation of Provo-level and Bonneville-level shoreline terraces, of Lake Bonneville, provide data for reanalysis of isostatic rebound in the Lake Bonneviile basin. Analysis of the differential rebound between the Provo shoreline (maximum rebound of 43 m) and the Bonneville shoreline (maximum rebound of 69 m) requires that the latter be an equilibrium shoreline. From the new data, the best estimate of the upper limit of effective viscosity of the uppermost mantle, assuming a half-space model and a 2000 year time interval between the Bonneville and Provo shorelines, is 2 x 1019 N sec m-2 (2 x 1020 poise). In addition, comparison of shoreline rebound profiles, for both shorelines, with theoretical plate flexure models indicates that the mean flexural rigidity of the Basin and Range lithosphere in this region is 1 x 1023 N m, or slightly less.
