LARGE IMPACT FEATURES ON EUROPA - RESULTS OF THE GALILEO NOMINAL MISSION

The Galileo Orbiter examined several impact features on Europa at cons iderably better resolution than was possible from Voyager. The new dat a allow us to describe the morphology and infer the geology of the lar gest impact features on Europa, which are probes into the crust, We ob serve two basic types of large impact features: (1) ''classic'' impact craters that grossly resemble well-preserved lunar craters of similar size but are more topographically subdued (e.g., Pwyll) and (2) very flat circular features that lack the basic topographic structures of i mpact craters such as raised rims, a central depression, or central pe aks, and which largely owe their identification as impact features to the field of secondary craters radially sprayed about them (e.g., Call anish), Our interpretation is that the classic craters (all <30 km dia meter) formed entirely within a solid target at least 5 to 10 km thick that exhibited brittle behavior an time scales of the impact events. Some of the classic craters have a more subdued topography than fresh craters of similar size on other icy bodies such as Ganymede and Calli sto, probably due to the enhanced viscous relaxation produced by a ste eper thermal gradient on Europa, Pedestal ejecta facies on Europa (and Ganymede) may be produced by the relief-flattening movement of plasti cally deforming but otherwise solid ice that was warm at the time of e mplacement, Callanish and Tyre do not appear to be larger and even mor e viscously relaxed versions of the classic craters; rather they displ ay totally different morphologies such as distinctive textures and a s eries of large concentric structural rings cutting impact-feature-rela ted materials. Impact simulations suggest that the distinctive morphol ogies would not be produced by impact into a solid ice target, but may be explained by impact into an ice layer similar to 10 to 15 km thick overlying a low-viscosity material such as water. The very wide (near antipodal) separation of Callanish and Tyre imply that similar to 10- 15 km may have been the global average thickness of the rigid crust of Europa when these impacts occurred. The absence of detectable craters superposed on the interior deposits of Callanish suggests that it is geologically young (<10(8) years). Hence, it seems likely that our pre liminary conclusions about the subsurface structure of Europa apply to the current day. (C) 1998 Academic Press.