Tectonic Processes on Europa: Tidal Stresses and Visible Features
R. Greenberg, P. E. Geissler, G. Hoppa, B. R. Tufts, D. D. Durda (LPL, U. Arizona), R. Pappalardo, J. W. Head (Brown U.), M. H. Carr (USGS), Galileo Imaging Team
Europa's orbital eccentricity, driven by the resonance with Io and Ganymede, results in ``diurnal'' tides (3.5 day period) and possibly in non-synchronous rotation. Both diurnal variation and non-synchronous rotation can create significant stress fields on Europa's surface and both effects may produce cracking. Patterns and time-sequences of apparent tectonic features on Europa include lineaments that correlate with both sources of stress, if we take into account non-synchronous rotation after initial crack formation, by amounts ranging up to several tens of degrees. For example, the stratigraphic time sequence of features in the Cadmus and Minos Linea region is consistent with the diurnal tensile-stress field, as it evolves during tens of degrees of non-synchronous rotation. Constraints on the rotation rate from comparing Voyager and Galileo images show that significant rotation requires greater than yr, but not necessarily longer than the age of the surface as indicated by the sparse craters. Once cracking is initiated, diurnal tides work cracks so that they open and close daily. Although the daily effect is small, over yr double ridges could plausibly be built along the cracks with sizes and morphologies consistent with observed structures, according to a model in which underlying liquid water fills the open cracks, partially freezes, and is extruded during the daily closing of the cracks. Thus several lines of observational and theoretical evidence can be integrated if we assume non-synchronous rotation and the existence of a liquid water layer.