RIFT FORMATION MECHANISMS IN THE ICY SHELL OF ENCELADUS

The icy crust of Enceladus is highly dissected by many forms of structural deformation including fractures, faults, and deformation belts. Some of the most prominent structural features observed on Enceladus are the three rift systems that emanate from the geologically active thermal anomaly of the south polar terrain (SPT). The rifts extend ~260 km northward in a fanning pattern away from cuspate protrusions along the tectonic boundary surrounding the SPT, producing geometries that motivate the descriptive term pinwheel rifts. North of the tectonic boundary and circumscribing the SPT are additional terranes of poorly-defined troughs and ridges with contrasting patterns of structural reworking, encircled by a second tectonic boundary that could be an indication of a past northern extent of the SPT. These relationships indicate that pinwheel rift development is strongly linked to the evolution of the SPT tectonic boundary and hence recent SPT geologic activity. The formation and driving processes behind rift systems through thick ice shells has yet to be studied in detail which is a major obstacle in understanding the structural processes that take place on icy satellites. In this study we use satellite imagery and shape-from-shadow measurements to compare the surface expressions of rift systems on Enceladus with the structural architecture and topography of potential analog rift systems in the thick lithospheres of both Earth and Mars. We use heave and throw calculations to produce topographic profiles across the rift systems on Enceladus, then use a comparative planetology approach to postulate the possible rift-driving processes that take place in the icy shell. Rift systems which propagate through thick lithosphere settings on Earth and Mars have been attributed to magmatic driving processes, such as dike intrusion. In the icy shell of Enceladus similar rifting processes could possibly be driven by cryomagmatism, such as the upwelling of warmer ice or icy slurries away from the SPT thermal anomaly.