Is There Evidence for Polar Wander on Europa?

Abstract

The orientations of the albedo lineaments, bands, and lineations on Europa's surface have been compared in previous studies with the global stress fields set up by orbital eccentricity, orbital recession, and nonsynchronous rotation. Of these orbital and rotational effects, nonsynchronous rotation, combined with an offsetting of the tidal bulge, comes closest to providing agreement between the stress field generated and the lineation orientations, if the lineations trace tension or extension fractures (McEwen 1986.Nature321, 49–51). However, inferred minimum principal stress directions for a broad region of wedge-shaped bands near the anti-Jove point cannot satisfactorily be accounted for by any of the stress fields above, but are consistent with the stresses resulting from a rotation of Europa's ice shell about an axis through the sub- and anti-Jove points, clockwise as seen from the anti-Jove hemisphere (P. M. Schenk and W. B. McKinnon 1989.Icarus79, 75–100). Calculations by Ojakangas and Stevenson (1989.Icarus81, 220–241) of the thermal state of Europa's ice shell indicate that spatial variations in the thickness of the shell may cause it to undergo such a reorientation. We have investigated whether any reorientation of the shell about an axis through the sub- and anti-Jove points produces a stress field consistent with the full, global set of prominent lineations on Europa's surface. We find that no such reorientation provides a good fit between the lineations and plausible fracture orientations derived from the principal stress trajectories. Topographic ridges, identified in a limited zone south of the anti-Jove point, are roughly consistent with compression due to clockwise polar wander, but the orientations of these ridges may be strongly biased by illumination direction. Within the limitations of the presently available imagery, nonsynchronous rotation is still the most likely cause of the prominant fractures on Europa's surface, and the best specific, albeit regionally limited, tectonic evidence consistent with recent polar wander remains the wedge-shaped bands.