Scalloped Depressions in Malea Planum, Southern Hellas Basin, Mars

Introduction: In order to investigate the effect of climate change on the surface morphology of Mars, we produced a detailed map of the location of scalloped shaped depressions, a type of dissected mantle terrain [1], located on the southern wall of the Hellas Basin between 50°E to 70°E longitude and 50°S to 70°S latitude. This is the area of Malea Planum that contains Amphitrites and Peneus Paterae. The region is covered by a latitude-dependent, several meters thick, surface mantle [2]. The mantle is presumably composed of dust and water ice thought to be related to obliquity-driven ice activity as recently as 2.1-0.4 Myr [3]. It has been suggested that scallops are formed by interstitial ice subliming from pore spaces in the mantle material [1, 4]. Scallops display north-south asymmetries with steep pole-facing scarps and gentler equator-facing slopes [5, 6]. We studied these features in this area to determine the recent geologic evolution of the region and to study if the regional climate of the Hellas basin has had an impact on the formation of these scallops. We also surveyed the southern hemisphere to determine the extent of these features for any longitudinal dependency. Data and Methods: Images from the High Resolution Stereo Camera (HRSC) on board ESA’s Mars Express mission were used to map scallop shaped depressions in the region. MOC-NA and THEMIS-VIS were used to supplement the mapping. MOLA data allowed depth measurements and cross sections of individual scallops. HiRise images were used to investigate scallops and characterize them to the limit of resolution. TES data supplied seasonal temperature variation measurements. THEMIS-IR daytime images were used to determine temperature gradients within scallops. Map of Scallops on the Southern Rim of Hellas: Scallops were observed in the study region almost exclusively between latitudes 52°S and 59°S. Fig. 1 shows our map of scallops. The light blue regions are where individual scallops or chains of scallops could clearly be defined and outlined. It can be seen that they contour the southern rim of the Hellas Basin. The areas where the scallops are most extensive and where we could measure their depths are along the highest topography of the rim between 1000 and -500 m altitude across our entire longitude range. Scallops then become smaller and shallower towards the basin center and are rare below -2000m. Scallops believed to be associated with the ejecta blanket of a large crater centered at 54.5°S, 65.6°E are responsible for the portion of the map found between -2500m and -4500m. No features interpreted as scallops were found in the region below -4500m. South of ~58°S scallops degrade quickly and are believed to have coalesced and eroded through the mantling material to an older layer. This is represented by the shaded green regions of the map south of the clearly defined scallops. Using MOLA shots, depths of individual scallops were measured and range between ~5m and ~40m. A radial pattern in the formation of scallops can be seen north of Peneus Patera, and scallops are often associated with the mantled ejecta blankets of craters in the region.