Distribution of strike-slip faults on Europa

Study of four different regions on Europa imaged by the Galileo spacecraft during its first 15 orbits has revealed 117 strike-slip faults. Europa appe ars to form preferentially right-lateral faults in the southern hemisphere and left-lateral faults in the northern hemisphere. This observation is con sistent with a model where diurnal tides due to orbital eccentricity drive strike-slip motion through a process of "walking," in which faults open and close out of phase with alternating right-and left-lateral shear, Lineamen ts that record both left-and right-lateral motion (e.g., Agave Linea) may r ecord the accommodation of compression in nearby chaotic zones. Nearly all identified strike-slip faults were associated with double ridges or bands, and few were detected along ridgeless cracks. Thus the depth of cracks with out ridges does not appear to have penetrated to the low-viscosity decoupli ng layer, required for diurnal displacement, but cracks that have developed ridges do extend down to such a level. This result supports a model for ri dge formation that requires cracks to penetrate to a decoupling layer, such as a liquid water ocean.