Tectonic Processes on Europa: Tidal Stresses, Mechanical Response, and Visible Features

Europa's orbital eccentricity, driven by the resonance with Io and Ganymede, results in "diurnal" tides (3.5-day period) and possibly in nonsynchronous rotation. Both diurnal variation and nonsynchronous 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 nonsynchronous rotation, after initial crack formation, by amounts ranging up to several tens of degrees. For example, the crosscutting time sequence of features in the Cadmus and Minos Linea region is consistent with a combined diurnal and nonsynchronous tensile-stress field, as it evolves during tens of degrees of nonsynchronous rotation. Constraints on the rotation rate from comparing Voyager and Galileo images show that significant rotation requires 10⁴yr, but could be fast enough to have allowed significant rotation since the last global resurfacing, even if such resurfacing was as recent as a few million years ago. Once cracking is initiated, diurnal tides work cracks so that they open and close daily. Although the daily effect is small, over 10⁵yr 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 nonsynchronous rotation and the existence of a liquid water layer.