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P.S. Mohit, B.T. Greenhagen, W.B. McKinnon (Washington Univ, St Louis)
The predicted apex-antapex asymmetry in crater density on Ganymede is not observed. We explore polar wander of a floating ice shell as a possible explanation. Due to the latitude dependence of absorbed solar radiation, the surface temperature at the poles is much less than that at the equator. As a result, a floating ice shell should be thicker at the poles. Assuming a conductive, elastic floating ice shell in equilibrium with interior radiogenic heat production, we calculate the resulting ice shell thickness differences. Using 30 and 5.3 mW/m2 as the heat flux at 4 Ga and at the time of maximum shell thickness, we find pole-equator thickness differences of ~15 km and ~85 km, respectively. Through their effect on the principal moments of inertia, these thickness variations may cause the shell to become unstable with respect to the rotation about the tidal axis. Because the “fossil” elastic bulge contributes to shell stability, the possibility of polar wander depends on shell rigidity. We find that degree-2 thickness variations are sufficient to destabilize the shell if its rigidity \mu < 0.27 GPa (for q = 30 mW/m2) or \mu < 1.4 GPa (for q = 5.4 mW/m2). For comparison, \mu of solid ice is 3.5 GPa, but the effective \mu of terrestrial sea ice is 0.33 ± 0.13 GPa. We also investigate the possibility of ductile flow in the shell reducing the thickness variations. Comparing the thermal time scale with the relaxation time of the shell thickness variations, we find strong thickness variations can only be maintained for higher heat flows. We conclude that polar wander may have occurred early in Ganymede's history or during an episode of strong tidal heating, when the shell would have been thin and weak. In order for polar wander to occur, however, the viscous resistance of the shell must be overcome, which is not taken into account in this simple model.
The author(s) of this abstract have provided an email address for comments about the abstract: psmohit@wustl.edu
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.