Basins and the South Polar Terrain of Enceladus: Hotspots vs. Coldspots, and More Evidence for Crustal Spreading and Convergence

Details Basins and the South Polar Terrain of Enceladus: Hotspots vs. Coldspots, and More Evidence for Crustal Spreading and Convergence Basins and the South Polar Terrain of Enceladus: Hotspots vs. Coldspots, and More Evidence for Crustal Spreading and Convergence

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.6109m&link_type=abstract

American Astronomical Society, DPS meeting #41, #61.09

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Stereo-derived topographic mapping of 50% of Enceladus reveals at least 6 large-scale, ovoid depressions (basins) 90-175 km across and 800-to-1500 m deep and uncorrelated with geologic boundaries (Schenk and McKinnon, GRL, in press). Their shape and scale are inconsistent with impact, geoid deflection, or with dynamically supported topography. Isostatic thinning of Enceladus’ ice shell associated with upwellings (and tidally-driven ice melting) can plausibly account for the basins. Thinning implies upwarping of the base of the shell of 10-20 km beneath the depressions, depending on total shell thickness; loss of near-surface porosity due to enhanced heat flow may also contribute to basin lows. Alternatively, the basins may overly cold, inactive, and hence denser ice, but thermal isostasy alone requires thermal expansion more consistent with clathrate hydrate than water ice. In contrast to the basins, the south polar depression (SPD) is larger ( 350 wide) and shallower ( 0.4-to-0.8 km deep) and correlates with the area of tectonic deformation and active resurfacing. The SPD also differs in that the floor is relatively flat (i.e., conforms roughly to the global triaxial shape, or geoid) with broad, gently sloping flanks. The relative flatness across the SPD suggests that it is in or near isostatic equilibrium, and underlain by denser material, supporting the polar sea hypothesis of Collins and Goodman [2007]. Near flatness is also predicted by a crustal spreading origin for the "tiger stripes” [e.g., Barr, 2008]; the extraordinary, high CIRS heat flows imply half-spreading rates in excess of 10 cm/yr, a very young surface age ( 250,000 yr), and a rather thin lithosphere (hence modest thermal topography). Topographic rises in places along the outer margin of the SPD correlate with parallel ridges and deformation along the edge of the resurfaced terrain, consistent with a compressional, imbricate thrust origin for these ridges, driven by the spreading.

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