Europa: Effects of rotation and tides on tectonic processes

Abstract

Tides due to orbital eccentricity may have a substantial effect on the rotation and tectonics observed on Jupiter's moon, Europa. A direct measurement of Europa's rotation rate has been made by measuring the positions of surface features relative to the terminator in both Voyager and Galileo images. From these measurements I have found that the rotation of Europa relative to the direction of Jupiter is <0.5° over a 17 year period, i.e. one rotation with respect to Jupiter would require at least 12,000 years. Non-synchronous rotation is also a significant source of global stress which, combined with the diurnal tidal stress, provides a failure mechanism resulting in tensile cracks on a global scale. The stress associated with rotational and diurnal tides can also explain the orientations and age relationships observed regionally in Europa's northern hemisphere. Additional global scale linear features also strongly correlate to these stress fields suggesting that they too may have also formed as cracks. After crack formation, diurnal tides may significantly affect the evolution of cracks through either ridges formation, regional extension, or strike-slip motion. The process of tidal shear displacement is analagous to actual walking. Mapping of five different regions on Europa has revealed 121 strike-slip faults. Based on these observations, Europa appears to support the formation of right-lateral faults in the southern hemisphere and left-lateral faults in the northern hemisphere. The theory of tidal walking predicts exactly that dichotomy on average over the hemispheres. Additionally, all of the mapped strike-slip faults were associated with double ridges and bands, but none were detected along cracks. Thus, cracks (even older ones) without ridges apparently have not generally penetrated to a decoupling layer, consistent with models for ridge formation that require cracks to penetrate to a liquid water ocean.