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R. Greenberg, G. Hoppa, B. R. Tufts, P. E. Geissler (LPL, Univ. of Arizona)
The characteristics of chaos regions on Europa suggest they may be sites of melt-through from below. They are wide-ranging in size, location and age. Most are similar to Conamara with a matrix reminiscent of frozen slush, and often rafts of pre-existing crust. Edges are of two types: ramps, perhaps the tapering of crustal thickness to zero, or cliffs where rafts appear to have broken clear from the shore. Of the lenticulae, "spots" generally appear to be small chaoses with textured matrix and occasional rafts, "pits" may represent incipient melting nearly at the surface, and many "domes" may be small chaoses raised by isostatic compensation following refreezing of the crust. The extent of chaoses often appears to be limited by ridge systems with the coastline parallel and set back by a distance comparable to the width of the ridge system. Pre-existing ridges often survive as causeways or chains of rafts. Boundaries of chaoses are apparently not controlled by pre-existing cracks, evidence that formation is essentially a thermal, rather than mechanical process. Evidently ridges thicken the crust such that melt-through is more likely between ridge systems. Subsequent cracks and ridges form across pre-existing chaoses: Recent chaoses have few cracks or ridges across them (with paths somewhat jagged as they meander among rafts), while the remains of old chaoses are only isolated rafts surrounded by densely ridged terrain. Thus two fundamental resurfacing processes have alternated over Europa's geological history: melt-through (at various places and times) forming chaos terrain, and criss-crossing by cracking and ridge-building forming densely ridged (and other tectonic) terrain. Mapping of chaos features based on morphology at 200 m shows that they correlate well with dark regions in global (2 km resolution) mosaics (except dark regions due to ridge margins or craters), so considerable area has been available to accommodate the expansion of crust that occurs along extensional bands and ridges. Chaos ubiquity suggests Europan geology has been dominated by the effects of having liquid water under a very thin ice shell, with chaos regions being widespread examples of zero shell thickness.