INVESTIGATING MIRANDA'S REGOLITH-COVERED CRATERS AND TECTONIC STRUCTURES: MANTLED BY RING PARTICLES, EJECTA FROM A LARGE IMPACT, OR PLUME DEPOSITS?

We are investigating whether a large-scale mantling event deposited regolith across Miranda’s cratered terrain. Miranda's cratered terrain consist of impact craters that fall into one of two extreme preservation states. "Mantled" craters have rims that appear to be relatively subdued, whereas rounded and "fresh" craters have relatively sharp rims. Miranda's mantled craters are confined to the cratered terrain while fresh craters are present within the cratered terrain and on the coronae. We analyzed both fresh and mantled craters in a region within the crater terrain, using images collected with the Voyager 2 Imaging Science Subsystem (ISS) and a digital elevation model that we created for this work. Our preliminary results show that depth-diameter ratios of mantled craters are smaller than “fresh” craters. We are continuing to investigate these results to constrain regolith thicknesses.

Miranda's tectonic structures also exhibit mantled and fresh morphologies. An example of a prominent mantled scarp is Argier Rupes, a large normal fault scarp which bounds Inverness Corona. Verona Rupes, nearby Argier, is a prominent example of a fresh scarp. In some locations fresh tectonic features exhibit trends consistent with mantled tectonic features, indicating that some sets of faults were likely active for long periods of time, with individual slip events both pre-dating and post-dating the emplacement of the mantling material. Polygonal impact craters in regions without visible fractures, which may be reflecting sub-regolith fractures, have been identified on Miranda. Analyses of Miranda’s polygonal impact craters indicate that a subtle and/or buried fracture system is present over the cratered terrain near Elsinore Corona.

The source of material mantling some craters and tectonic structures on Miranda remains uncertain. One possibility is that this material is sourced from Uranus’ rings, in particular the dusty and diffuse mu-ring, which could be sustained by particles ejected from the surface of the embedded ring moon Mab. Alternatively, plume deposits resulting from cryovolcanic activity, possibly associated with coronae formation, could have mantled Miranda’s surface. It is also possible that ejecta from a basin-forming impact on Miranda mantled the cratered terrain.