TIDALLY DRIVEN COULOMB FAILURE CONDITIONS OF STRIKE-SLIP FAULTS ON ENCELADUS AND EUROPA

We investigate the conditions necessary for tidally driven tectonic faulting on Enceladus, and consider implications for Europa, using methods derived from diurnal tidal stress models and terrestrial earthquake physics. For Enceladus, our critical objectives are to understand and compute Coulomb failure conditions to assess the variability of tiger stripe fault failure (in both time and space) throughout the satellite’s orbital cycle. We explore a suite of model parameters (fault orientation, frictional coefficient, fault depth, and ice shell thickness) that inhibit or promote shear failure, and integrate these conditions into a 3D time-dependent fault dislocation model to evaluate tectonic displacements and stress variations at depth during a tiger stripe orbital cycle. Depending on the sequence of stress accumulation and subsequent fault slip, for a 24 km ice shell underlain by a global subsurface ocean, we estimate that the tiger stripe fault segments can accumulate up to ~70 kPa of resolved shear stress, at shallow fracture depths (2-4 km), before meeting the Coulomb failure criterion and generating a strike-slip event. For these events, we find that low coefficients of friction (μ_f = 0.1-0.2) most readily permit shear failure along the tiger stripe faults, and that right- and/or left-lateral slip responses are possible. Unlike Enceladus, diurnal tidal stresses on Europa may be insufficient to cause strike-slip fault motion at shallow fault depths. Because such motion would be expected, given the many observed strike-slip offset features on Europa (for example, Agenor Linea), we consider the role of non-synchronous rotation tidal stresses as secular stress sources for major strike-slip faulting. Preliminary application of the Coulomb failure criterion, assuming μ_f = 0.2 and a fault depth of 6 km, reveals that a combination of non-synchronous rotation and diurnal tidal stresses are required for Agenor Linea to succumb to right-lateral shear failure at specific portions of the orbital cycle. Together, these tidally driven failure models for Enceladus and Europa are providing key insights into the frictional and material properties, and their variation with depth, of active fault systems throughout the solar system.