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R. Greenberg, B.R. Tufts, G.V. Hoppa, P. Geissler, J. Riley (LPL, U. Arizona)
The tidal-tectonic processes that appear to have governed Europa's geology require an ocean linked to the surface through a very thin crust, and provide a variety of evolving environmental niches, hospitable to life. The two dominant surface renewal processes that have alternated at various places and times throughout the geological history are melt-through (producing chaotic terrain) and tectonics (making cracks, ridges and bands); either process erases what was there before. Sequences and orientations of regional scale lineaments correlate with tidal stress patterns suggesting that there is a liquid ocean under the ice crust and rotation is non-synchronous. Subsequent diurnal tidal working of cracks creates ridges, by pumping liquid from below, and squeezing or spattering slush to the surface, with some distortion of the lip of the crack by the daily pounding. Strike-slip displacement and cycloidal cracking are driven by diurnal tides. Cracks can form bands if dilated by regional extension, perhaps also driven by tides. The same model of liquid water under a thin ice shell explains the characteristics of chaos areas as the result of melt-through from below. Chaos areas are wide-ranging in size (up to 1300 km across), location, and age. Pre-existing ridges resist melt-through and can form chaos boundaries or causeways or chains of rafts. Subsequent cracks and ridges can mask older chaos areas. Melt-through explains how compression can accommodate dilation along bands without leaving a trace. Surface colorants correlate with locations where ocean water reached the surface. As a result of tides, liquid water regularly bathed crustal cracks and surfaces with heat and whatever nutrients are included in the oceanic chemistry, creating a variety of potentially habitable crustal environments, as well as addressing requirements for life in the ocean itself. The processes described here were recent, and thus may continue today. In any particular niche, conditions have gradually changed, providing any organisms with the need for adaptation and opportunity for evolution.
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The author(s) of this abstract have provided an email address for comments about the abstract: greenberg@lpl.arizona.edu