Mathematics – Logic
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.2404b&link_type=abstract
American Astronomical Society, DPS meeting #38, #24.04; Bulletin of the American Astronomical Society, Vol. 38, p.522
Mathematics
Logic
Scientific paper
Voyager and Cassini images have revealed a myriad of extensional features on Enceladus’ surface including cracks, graben, and curvilinear fractures. In addition, networks of sub-parallel, periodic ridges and troughs exist both in regions formerly identified as "smooth plains” in Voyager 2 images, and in the regions north of Samarkand Sulcus. In general, such features have topographic amplitudes greater than 200 m and wavelengths on the order of 10 km. These regions bear a striking similarity to Ganymede's grooved terrain, suggesting that the two terrain types may have formed in an analogous manner: unstable extension of the lithosphere that deforms into periodically spaced pinches and swells. We examine such unstable extension with a fully nonlinear, finite-element model under conditions appropriate to Enceladus. Our model incorporates the relevant rheological parameters for an ice lithosphere and allows elastic, viscous, and plastic deformation. These models help constrain the conditions necessary for the formation of the ridge-and-trough terrains. We find that unstable extension in Enceladus’ low-gravity environment produces relatively strong instability growth when compared to instability growth on larger, high-gravity bodies such as Ganymede. The ridges and troughs formed by such extension have amplitudes of several hundred meters and topographic wavelengths between 2 km and 28 km. Assuming large strains ( 30%) are locally available, instability growth is most effective at low thermal gradients ( 5 - 10 K km-1) and moderate strain rates (10-13 s-1). These results contrast strongly with the predictions of linear, infinitesimal strain models, which suggest that maximum lithospheric deformation occurs when thermal gradients are high. Our results also suggest that the formation of extensional instabilities is capable of partially resurfacing preexisting topography 10 m to 100 m in amplitude, thus helping to explain the relative paucity of craters within the ridge-and-trough terrain. This research is supported by NASA PG&G grant #NNG04GI46G.
Bland Michael T.
Showman Adam P.
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