Abstract
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 Conamara with a matrix reminiscent of frozen
slush and often rafts of preexisting 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. The small
features called lenticulae generally appear to be small chaoses 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 the 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 preexisting cracks, consistent with formation by
a thermal, rather than mechanical, process. Ridges may thicken the
crust such that melt-through is more likely (but not always) between
ridge systems. Subsequent cracks and ridges form across preexisting
chaoses, ranging from fresh cases with few cracks or ridges 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 fundamental resurfacing processes have alternated over Europa's
geological history: melt-through (at various places and times) forming
chaos terrain, and tectonic cracking and dilation building densely
ridged and banded terrain. Mapping of chaos features based on morphology
at 200 m shows that they correlate, albeit imperfectly, with dark
regions in global (2-km resolution) mosaics (except dark regions
due to ridge margins or craters). Extrapolating from our mapping
of the 5% of Europa covered by appropriate images, at least 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 overprinted by tectonic structures. Considerable area has been
available globally to accommodate the expansion of crust that occurs
along extensional ridges and bands. Chaos ubiquity suggests that
Europan geology has been dominated by the effects of having liquid
water under a very thin ice shell, with chaos regions being widespread
indicators of occasional zero shell thickness. (C) 1999 Academic
Press.
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