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W.B. McKinnon (Dept. EPSc and McDonnell Center for Space Sci., Washington Univ., St. Louis MO 63130, USA)
Much evidence points to the existence of an ocean on Europa, but the thickness of the surface ice shell is hotly debated. The discovery of folds on Europa by Prockter and Pappalardo (Science 289, 941, 2000) allows in principle a rigorous determination of local heat flow, given that the logical cause, viscous buckling, has been studied in a terrestrial context for decades. The wavelengths observed imply a folding layer thickness probably \gtrsim 10 times thinner (with adjustment for how precisely one models the brittle-ductile transition, or BDT), or \lesssim 2 km at the Astypalaea site (to the BDT). In compression, and assuming prefractured ice of zero cohesion, this implies ~ 5 MPa of differential stress at the BDT. This requires 10s of deg of nonsynchronous rotation (as noted by Prockter and Pappalardo), but the strain rates are probably low (~ 2 x 10-17 s-1 based on theoretical models of shell rotation). The high latitude of Astypalaea implies a lower than average surface temperature, however, so the heat flow implied from the strain rate and BDT above is large, \gtrsim 125 mW m-2 (and grain-size effects are modest). Well-defined ring graben around Tyre and Callanish have widths between ~ 0.75 and 1.5 km. Assuming the master faults of these graben intersect at the extensional BDT and that the appropriate strain rate is much greater than above, conservative assumptions for the latter yield heat flows of ~ 240 mW m-2 for a 1-km thick ``impact lithosphere." These estimates can be added to those discussed in Pappalardo et al. (JGR 104, 24,105, 1999). The above imply conductive shell thicknesses no greater than ~ 5 km, but the heat flows are so large in this case that they would have to be core derived. Alternatively, the heat flows are supplied by a tidally heated, convecting sublayer, and the total shell thickness is greater.
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