EFFECTS OF NONSYNCHRONOUS ROTATION STRESSES ON THE DEFORMATIONAL HISTORY OF ENCELADUS'S SOUTH POLAR TERRAIN

The south polar terrain (SPT), centered on the south pole of Saturn's moon Enceladus, is a young, highly fractured region ~350 km across, that is the source of numerous eruptive plumes of water-ice and a high energy flux. Previous studies have focused on the largest fractures in the region, the "tiger stripes," and their present-day activity, which is likely driven primarily by diurnal tides. However, there are numerous other fractures in the SPT that can be grouped, along with the tiger stripes, into distinct sets based on their relative age and orientation. These fracture sets point to a long and ordered history of fracturing in an ice shell that has progressively rotated about the south pole, and provide evidence that the moon may have experienced stresses due to long-term nonsynchronous rotation (NSR) in addition to diurnal tidal stress. The smaller (tens to hundreds of kPa) diurnal tidal stresses are enough to create shearing along existing fractures, but may be insufficient to overcome the tensile strength of ice and induce fracturing. We use a 4-layer (inner core, global liquid layer, less viscous lower ice layer, and a more viscous upper ice layer) viscoelastic model to calculate the diurnal and NSR stresses for a variety of ice shell thicknesses and rheologies. We find that in the SPT, where the ice shell is likely thinner than the rest of Enceladus, the NSR stresses can be sufficiently greater in magnitude than the diurnal tidal stresses to overcome the tensile strength of the ice. Our modeling shows that the magnitude of the NSR stress is dominantly controlled by the thickness of the more viscous (i.e., elastic/brittle) upper ice layer. For NSR periods of between 0.1 and 2 Myr and an upper ice layer thickness of between 2 and 4 km, the NSR stress is an order of magnitude larger than the diurnal stress. However, for the same NSR rates but an upper ice layer thickness between 6 and 8 km, the NSR stress drops to a value nearly equivalent to the diurnal stress. The dependence of the magnitude of NSR stress on the thickness of the upper ice layer could help explain why young fracturing is only observed in the SPT. The thinner ice at the SPT would experience higher stresses and be more prone to fracturing, whereas outside the SPT, the theorized thicker ice shell would be less prone to fracturing.