Polygonal Impact Craters on Miranda and Ariel Reveal Hidden Tectonism

Tuesday, 15 December 2020
Chloe Beddingfield, NASA Ames Research Center, Moffett Field, CA, United States; SETI Institute Mountain View, Mountain View, CA, United States and Richard J. Cartwright, SETI Institute, Mountain View, CA, United States
We mapped the spatial distributions and orientations of polygonal impact craters (PICs) on the classical Uranian satellites Miranda and Ariel to gain insight into the complex tectonic histories of these moons. PIC straight rim segments in regions without visible fractures can be used to infer sub-regolith and/or sub-resolution fracture orientations. On Miranda, PICs are located on the trailing hemisphere, overprinting Elsinore Corona and the nearby surrounding cratered terrain. These PICs do not follow the E-W orientations of the ridges within Elsinore’s bounding terrain, indicating that these ridges are not underlain by deep-rooted, sub-vertical normal or strike-slip faults. Instead, the ridges may be contractional features, consisting of folds and possibly thrust faults, or cryovolcanic features without fault structures. Furthermore, the PICs in this region indicate that a NW oriented fracture system is present over much of Miranda’s trailing hemisphere. Another PIC, somewhat distal from Elsinore on Miranda’s anti-Uranus hemisphere, might be associated with the NW fractures as well. Additionally, three PICs are present proximal to Verona Rupes on Miranda’s Uranus-facing hemisphere. These PICs exhibit N-S orientations consistent with the surrounding fractures that make up Miranda’s Global Rift System. One other PIC is proximal to Arden and might be associated with the Arden bounding terrain. Based on cross-cutting relationships with Miranda’s ancient subdued crater population, the NW fracture system is older than Elsinore. However, this fracture system also overprints Elsinore, suggesting reactivation of the NW fractures after this corona had formed. On Ariel, PICs with NW-SE orientations are present in the Cratered Plains in between the large canyons, Sylph, Korrigan, and Pixi Chasmata. The orientations of these PICs suggest that a previously unrecognized fracture system is present within this terrain. The inferred fractures are sub-parallel to Kra Chasma, supporting the interpretation that this feature is bound by fault scarps. Our results suggest that some tectonic processes modified regions of Miranda and Ariel before the formation of many craters.