Constraining Enceladus’s ice shell thickness using tidally driven Coulomb failure models of the tiger stripe fractures

The thickness of Enceladus’s ice shell, and subsequently the depth of its underlying ocean, plays an important role in determining the magnitude and orientation of tidal stresses on the surface of the satellite. The objective of this study is to attempt to constrain Enceladus’s ice shell thickness through assessment of tidally driven Coulomb failure of the tiger stripe fractures, probable sources of tectonic activity that reside along Enceladus’s south polar region. Using the SatStress computational model and assuming a global ocean and uniform ice shell thickness, we compare tidal stresses resulting from a suite of models of variable ice shell thickness (6 km, 24 km, 30 km, 40 km, 50 km, 75 km, and 90 km). Evaluation of the Love numbers for each model (l2, h2 and k2) suggest that ice shell thicknesses greater than a few 10s of km are not capable of generating stress conditions conducive to fault failure on Enceladus. We further investigate the feasibility of tiger stripe failure by applying a Coulomb failure model to the resulting tidal stresses. Based on previous work, we have shown that a 24 km thick ice shell model is capable of generating stress conditions that promote Coulomb failure and strike-slip displacements of ~0.4 m along some portions of the tiger stripe system, assuming a coefficient of friction of 0.2. We compare this reference model to the 6 km, 30 km, 40 km, 50 km, 75 km, and 90 km ice shell models and find that thin to moderate ice shell models do indeed support fault failure, while thick ice shell models do not. Models based on a 6 km ice shell thickness can yield strike-slip displacements on the order of 1 m along all segments of the tiger stripe system. Models based on a 30 km ice shell thickness can also generate strike-slip displacements (~0.1 m), however only on isolated fault segments. Finally, models based on a 40 km ice shell thickness or greater cannot generate stresses capable of Coulomb failure along any of the tiger stripe segments. While it is possible that ice shells thicker than 30 km may support Coulomb failure along the tiger stripe fractures given a reduced coefficient of friction (< 0.2), strike-slip displacements and overall fault activity will be substantially reduced in comparison with tectonic activity estimated by thinner ice shell models.