CENTRAL PIT AND DOME CRATERS - EXPOSING THE INTERIORS OF GANYMEDE ANDCALLISTO

Central pit craters on the large icy satellites Ganymede and Callisto are an unusual crater class perhaps related to the unusual properties of water ice. The detailed morphology and dimensions of pit crater fea tures differ from those of smaller, normal, complex craters on these t wo satellites. Pit craw depths appear to be constant regardless of dia meter. Rim collapse is also restricted relative to that in lunar crate rs or smaller Ganymede craters. The floors of pits in craters larger t han approximately 60 km are occupied by smooth bright domes. Pit-to-cr ater and dome-to-crater diameter ratios increase linearly with crater diameter, and are indistinguishable on Ganymede and Callisto and with terrain type or surface longitude. The floors and central structures o f young, bright-rayed pit craters such as Osiris are covered by a thin uniformly bright impact melt or frost deposit. Thus, domes and pits f orm rapidly, on the time scale of the impact itself, rather than by lo ng-term, postimpact intrusion or extrusion. The bright domes in pit cr aters are most simply explained as the uplift and exposure of relative ly ice-rich material from depths of approximately 3.5 to 5 km during i mpact. This process is almost directly analogous to the uplift and exp osure of anamolous deep-seated material observed in terrestrial and lu nar craters. The unusual pit morphology on icy satellites may be the r esult of impact into crust that is mechanically much weaker at shallow depth than on rocky bodies such as the Moon. Because crater morpholog y is strongly dependent on ice-rock composition, the similarity of pit and dome dimensions on Ganymede and Callisto indicates that the struc ture and rheology of the crusts of these bodies (down to depths of app roximately 10 km) are very similar and have been for several b.y. Pit crater morphology indicates that the crusts of both satellites are pro bably ice-rich and differentiated.