FEEDBACK BETWEEN ICE SHELL THINNING AND NONSYNCHRONOUS ROTATION STRESSES DRIVES FRACTURING IN THE SOUTH POLAR REGION OF ENCELADUS

Numerous studies of the south polar region of Saturn’s moon Enceladus have focused on the four main fractures (the tiger stripes), their associated water plumes, and the effects of diurnal tidal stresses on these fractures. However, due to their small magnitude, diurnal stresses alone are unable to completely explain the initial formation of the tiger stripes and the many additional fractures in the region. We suggest nonsynchronous rotation (NSR) stresses, induced by a freely rotating ice shell over a global ocean, are large enough to overcome the tensile strength of the ice shell and initiate fracturing on Enceladus. Using the tidal stress calculation program SatStressGUI, we demonstrate the dependence of the magnitude of the NSR stress on the thickness of the ice shell, particularly the brittle outer layer of the ice shell. We suggest the present-day brittle layer in the south polar region is 3-4 km thick whereas in the more northern regions, it is 6-8 km. The difference is most likely due to the elevated energy flux at the south pole, compared to the regions to the north, resulting in a warmer and thinner ice shell in the south polar region. Our geologic mapping of the south polar terrain has revealed four sets of fractures of varying orientations centered near the south pole. The angles separating the orientations of the fracture sets decreases from the oldest set to the youngest one (which includes the tiger stripes). The decreasing angular separation between fracture patterns suggest an ice shell that has weakened in strength over the course of its recent geologic history; this could be a result of increased heating, thinning of the ice shell, or a combination of the two. As the ice shell thinned, the NSR stresses were enhanced making fracturing more likely after a smaller amount of NSR compared to the previous fracturing episode. We suggest a 2-4 km thinning of the ice near the south pole over the past ~500,000 yrs could account for the angular separation between the fracture sets.