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J. M. Moore, E. Asphaug, D. Morrison (NASA Ames), R. J. Sullivan (Cornell), C. R. Chapman (SWRI), R. Greeley, J. E. Klemaszewski, S. Kadel, F. Chuang, J. Moreau, K. K. Williams (ASU), P. E. Geissler, A. S. McEwen, E. A. Turtle, C. B. Phillips, B. R. Tufts (LPL, U. Arizona), J. W. Head, R. T. Pappalardo, G. C. Collins (Brown), G. Neukum, R. Wagner (DLR, Germany), K. P. Klaasen, H. H. Breneman, K. P. McGee, D. A. Senske (JPL), J. Granahan (U. Hawaii), M. J. S. Belton (NOAO), Galileo SSI Team
The Galileo Orbiter, during the GEM phase of this mission, has examined a number of impact features on Europa at considerably better resolution and coverage than was possible from either Voyager or during the Galileo nominal mission. The new data allow us to describe the morphology and infer the geology of the largest impact features on Europa, which are probes into the crust. The GEM observations allow us to construct a suite of primary impact features on Europa; a comprehensive "family" portrait and ordering (by size on one axis and morphologic variations within a given size bin along the other). We have also made detailed description of individual impact features including topography (from stereo), crater-related materials deposits, cross-cutting relations, and material-related color variations. We observe 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 impact craters such as raised rims, a central depression, or central peaks, and which largely owe their identification as impact features to the field of secondary craters radially sprayed about them (e.g., Callanish). One of our preliminary conclusions are that Callanish and Tyre display non-"classic" impact features morphologies and a series of large concentric structural rings cutting impact-feature-related materials. Impact simulations suggest that Callanish and Tyre would not be produced by impact into a solid ice target, but may be explained by impact into an ice layer ~10 to 15 km thick overlying a low viscosity material such as water.
The author(s) of this abstract have provided an email address for comments about the abstract: jmoore@ringside.arc.nasa.gov