Chaos on Europa

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. The largest are hundreds of kilometers across. Most are similar to Conamar a with a matrix reminiscent of frozen slush and often rafts of preexisting crust. Edges are of two types: ramps, perhaps the tapering of crustal thick ness to zero, or cliffs, where rafts appear to have broken clear from the s hore. The small features called lenticulae generally appear to be small cha oses with textured matrix and occasional rafts, 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 t he coastline parallel and set back by a distance comparable to the width of the ridge system. Preexisting ridges often survive as causeways or chains of rafts. Boundaries of chaoses are apparently not controlled by preexistin g cracks, consistent with formation by a thermal, rather than mechanical, p rocess. Ridges may thicken the crust such that melt-through is more likely (but not always) between ridge systems. Subsequent cracks and ridges form a cross preexisting chaoses, ranging from fresh cases with few cracks or ridg es across them (with paths somewhat jagged as they meander among rafts) to heavily dissected examples. Isolated tilted raft-like blocks surrounded by densely ridged terrain may be relies of former chaotic terrain. Thus two fu ndamental resurfacing processes have alternated over Europa's geological hi story: melt-through (at various places and times) forming chaos terrain, an d tectonic cracking and dilation building densely ridged and banded terrain . Mapping of chaos features based on morphology at 200 m shows that they co rrelate, albeit imperfectly, with dark regions in global (2-km resolution) mosaics (except dark regions due to ridge margins or craters). Extrapolatin g from our mapping of the 5% of Europa covered by appropriate images, at le ast 18% of the surface of Europa is fresh appearing chaos, an additional 4% is slightly modified chaos, and much more older chaotic terrain has been o verprinted by tectonic structures. Considerable area has been available glo bally to accommodate the expansion of crust that occurs along extensional r idges and bands. Chaos ubiquity suggests that europan geology has been domi nated by the effects of having liquid water under a very thin ice shell, wi th chaos regions being widespread indicators of occasional zero shell thick ness. (C) 1999 Academic Press.