UNRAVELING THE FRACTURE HISTORY OF THE SOUTH POLAR TERRAIN ON ENCELADUS

Four main fractures dominate the south polar region of Saturn’s icy moon, Enceladus. The factures have fairly consistent orientations and are commonly known as “tiger stripes.” Although these cracks are the source of the geyser-like eruptions imaged by the Cassini spacecraft, they are not the only systematic fracture set in the area. Our detailed mapping has revealed three additional relict fracture sets that have tiger stripe-like characteristics but different orientations. Using crosscutting relationships, relative ages were established for the four systematic fracture sets, which show a counterclockwise progression in orientation through time. These fracture sets imply that the causal south polar terrain (SPT) stress field created distinct fracture sets at different points in time, gradually rotating ~153° counterclockwise relative to the present day surface and culminating in the stress field that produced the tiger stripes. Older fracture sets influenced the development of younger sets; the most obvious examples are found within the tiger stripes. The tiger stripe Alexandria Sulcus inherited a portion of an older set along part of its length, creating a jog along strike. Eruptive plume locations appear to correlate with intersections between the tiger stripes and older fracture sets, suggesting a structural control on sites of eruptions. The consistent orientations of the four fracture sets, combined with the temporal change in orientations, is compatible with the process of nonsynchronous rotation being the primary cause of fracturing. If the shell slowly rotated eastward, distinct fracture sets may have formed at different points in time in the SPT, superimposing relatively older, rotated sets. Such a process may hint at the presence of a global liquid ocean on Enceladus that decouples a mobile ice shell from the silicate interior.