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G. V. Hoppa, B. R. Tufts, R. Greenberg, P. E. Geissler (LPL, Univ. of Arizona), and the Galileo Imaging Team
Diurnal tides due to orbital eccentricity may drive strike-slip motion on Europa through a process of "walking" [1,2]. Along Astypalaea Linea ~42 km of right-lateral displacement has resulted from walking, as follows [1,2]. Every 3.5 days, tidal stress across the fault alternates between tension and compression. Tidal shear displacement along Astypalaea is right-lateral while the fault is under tension. During the compressive phase of the diurnal cycle no lateral motion can occur because the fault is closed. Inelastic behavior of the ice shell prevents it from moving back to its original position when tension reopens it during the next cycle.
Mapping of five different regions on Europa has revealed 121 strike-slip faults. At high southern latitudes near Astypalaea all of the strike slip faults identified were right-lateral. Closer to the equator in the southern hemisphere, nearly 95% in the wedges region were right-lateral. Very close to the equator in the northern hemisphere, near Conamara chaos, the distribution is equal between left-lateral and right-lateral faults. At 23 north of the equator (bright plains), 80% left-lateral. Further north at 35, 75% left-lateral. Based on these observations, Europa appears to preferentially 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. At the five locations, non-synchronous rotation [3] explains the azimuthal orientations and distribution of sense of shear by allowing strike-slip faults to form between 60 to 90 west of their current position. Alternatively, stress due to differential rotation might also explain the observed shear patterns.
Additionally, all of the mapped strike-slip faults were associated with double ridges and bands, but none were detected along cracks (even older ones). Thus, cracks without ridges apparently have not generally penetrated to a decoupling layer, consistent with the model [2] that ridges form only along cracks that penetrate to a liquid water ocean.
References: Tufts et al. 1997. Bull AAS, 29, 3, 983; Greenberg et al. 1998. Icarus, in press. Greenberg et al. 1984. Icarus 58, 186.