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P.M. Schenk (LPI, Houston Tx)
As the next great voyage to Europa founders in a chilly sea of programmatic icebergs, the great thick-crust thin-crust debate over the thickness of Europa’s putative floating ice shell continues unabated. Completion of stereo and 2-D photoclinometric mapping covering ~20% of the surface of Europa reveals a variety of topographic features. These geophysical data place important constraints on geologic processes and shell thickness. Impact crater topography reveals two abrupt breaks in Europa's depth/Diameter curve, the second of which (at ~30 km diameter) indicates a minimum ice shell thickness of ~20 km (Schenk, Nature, 417, 419, 2001). Matrix material within chaos regions lies at or above the topographic level of surrounding plains. This is more consistent with a diapiric origin of chaos (rather than a melt-through origin). Matrix material is also variable and domical over 10's of km scales. This may reflect the topography of individual diapirs, the spacing of which suggests a thickness ofi >15 km (Schenk & Pappalardo, submitted). The total topographic range on Europa may be ~2 km, much greater than usually assumed. Such relief is more consistent with a thick crust. Maximum positive relief is associated with ~100 oblong plateaus 10-40 km across and up to 1 km high. Negative relief occurs as elliptical pits or elongate troughs 5-10 km across and up to 500 m deep. Irregular depressions appear to be up to 1 km deep. Simple buoyancy arguments indicate that the crust must be 6-8 km thick to preserve such negative relief against melt-through (Schenk & McKinnon, in prep.). Although older ridged plains could have formed in a thinner crust, the topography of most other Europan geologic terrains of various ages is consistent with a thick ice shell has been at least 6-8 km thick and probably on the order of 20 km thick for much of its history.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.