Impact features on Europa: Results of the Galileo Europa Mission (GEM)

During the Galileo Europa Mission (GEM), impact features on Europa were obs erved with improved resolution and coverage was compared with Voyager or th e Galileo nominal mission. We surveyed all primary impact features >4 km in diameter seen on Europa (through orbit E19). The transition from simple to complex crater morphology occurs at a diameter of about 5 km. We calculate d the transient crater dimensions and excavation depths of all craters surv eyed. The largest impact feature (Tyre) probably had a transient crater dep th between 5 and 10 km and transported material to the surface from a depth of not greater than similar to4 km. Craters < 30 km in diameter, such as M anannan and Pwyll, formed within targets whose immediate subcrater material s exhibited nonfluid behavior on time scales of the impact event, and that are capable, especially in the case of Pwyll, of supporting significant loc al topographic loads such as a central peak. These craters are nevertheless quite shallow, with very subdued floors, and we suspect that Manannan and Pwyll's small depth-to-diameter ratios are due to the isostatic adjustment of large-scale topography, facilitated by warm, plastically deformable ice at depth. Morphological similarities between Callanish and Tyre strongly im ply that conclusions reached regarding Callanish in J. Moore et al. (1998, Icarus 135, 127-145) also apply to Tyre, which was that Callanish is the co nsequence of impact into target materials that are mechanically very weak a t depth. New evidence that Callanish's circumferential rings formed before the proximal ejecta became immobile implies a low-viscosity substrate at th e time of impact. We also report additional evidence that a component of th e proximal ejecta of Callanish was emplaced as a fluid. Our observations of Pwyll secondaries support the conclusions stated in Alpert and Melosh (199 9) that impacts on icy bodies eject smaller fragments and that fragment siz e decreases more gradually as velocity increases than observed for impacts on silicate bodies at equivalent ejection velocities. Examination of Pwyll' s secondary craters reveals azimuthal variations, with ejecta fragment size s being larger near the center of a ray than off the ray. Our initial analy sis of the characteristic size distribution of Pwyll's secondary craters sh ows that they form a differential slope slightly shallower than -4, Similar steep slopes for small craters on Ganymede imply that small craters there are mostly formed by secondary impact, and the jovian system may thus be de ficient in small impacts relative to the environment of the terrestrial pla nets. (C) 2001 Academic Press.