The present-day icy surface of Europa is covered with large ridges and cracks, and some of these surface features are not well explained by the stress field predicted to arise from the diurnal variations in tidal forces alone. Past work suggests the existence of a large background stress field, which could be due to non-synchronous rotation (NSR) of the ice shell , ice shell thickening , or polar wander . However, direct evidence for NSR has been lacking due to the slow hypothesized timescale and the limitations of available data. In addition to this potential for past or ongoing ice shell reorientation, Europa possesses a hemispheric color asymmetry due to irradiation of the icy surface by high-energy particles entrained in the Jovian magnetosphere . Multispectral studies with Voyager data revealed a global asymmetry in albedo and color between the leading and trailing hemispheres, particularly in the relative ultraviolet (UV) spectral reflectance . Given an exogenic origin for this alteration, reorientation of the surface by NSR can in principle be detected and estimated by searching for a characteristic mismatch – a “smear” – in the longitudinal distribution in present-day UV reflectance variations on Europa’s surface, relative to the predicted pattern for synchronous rotation. Here we present results of three models of exogenic surface discoloration, which are used for calculating constraints on the NSR rate of Europa. Two of the models are observational, with one representing an actively discoloring surface, and the other assuming a steady-state. A third higher-fidelity physical model of ice discoloration is developed to validate the observational models. Decomposing the longitudinal ultraviolet and visible color variations from Voyager into sine and cosine terms, and using the three discoloration models, we find no detectable signature of NSR. The models are also used to provide predictions of the scale of the expected signature for various levels of NSR previously suggested.
References:  Greenberg, R. et al., (2002), The rotation of Europa. Celest. Mech. Dyn. Astron. 83, 35-47  Nimmo, F., (2004), Stresses generated in cooling viscoelastic ice shells: application to Europa. J. Geophys. Res. Planets 109  Schenk et al., (2008), True polar wander on Europa from global scale small-circle depressions. Nature 453, 368-371  Carlson et al., (2009), Europa’s surface composition. Europa 283-327  Nelson et al., (1986), Europa: characterization and interpretation of global spectral surface units. Icarus 65, 129-151