DPS 34th Meeting, October 2002
Session 41. Europa
Oral, Chair(s): W. McKinnon and E.B. Bierhaus, Friday, October 11, 2002, 10:45am-12:05pm, Room M

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[41.03] The Evil Twin of Agenor, and other issues regarding crust displacement on Europa

R. Greenberg, P. Geissler, D. O'Brien, T. Hurford, G. Hoppa (LPL, U. Arizona)

Convergence: Diametrically opposite Agenor, a similar bright linear feature has been identified, surrounded by markings that allow reconstruction, which shows it to be a convergence feature. This result supports the suggestion by Schenk and McKinnon that Agenor is a site of surface convergence, and complements the discovery of other similar convergence locations by Sarid et al. The past difficulty in identifying convergence sites (needed to balance the creation of new surface at divergence bands) was due to the fact that these features are subtle and do not exhibit structures expected from experience on solid planets.

Divergence: Two major, parallel, but widely separated dilation bands represent response to driving forces that acted over a large region, according to reconstruction of the terrain between them. These bands were not formed by local forces acting separately across each band. This result helps constrain the still unknown driving mechanism for dilation (and for corresponding convergence).

Shear: Strike-slip displacement, common on Europa, is explained by Hoppa et al.'s diurnal tidal walking mechanism,which requires that a crack penetrate elastic ice to a low-viscosity decoupling layer, e.g. the liquid ocean. It has been suggested that convecting viscous ice under the colder elastic layer would provide adequate decoupling. Energy considerations provide a preliminary way to explore the plausibility of that idea. Tidal deformation defines a required net strain on the crust. In a simple model with a thick underlying viscous ice layer, the work done by the tide to deform the crust is minimized if the elastic layer distorts, rather than shear along a preexisting fault. Strike-slip is only energetically preferable if the viscous layer under the elastic zone is thin. In that case the concentrated viscous heating at the bottom of the crack would likely be sufficient to melt an opening between the ocean and the crack.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.