POLYGONAL IMPACT CRATERS AS AN INDICATOR OF FRACTURING - AN EXAMPLE FROM GREATER HELLAS REGION, MARS. T. Öhman

Introduction: Polygonal plan view is a long known [e.g. 1,2,3] but largely forgotten feature of impact craters. The aim of this study is to map the distribution of polygonal craters in the greater Hellas region, Mars, and to analyse whether or not polygonal craters can be used as a tool in the investigation of the structural and tectonic features of a planet's crust. One goal of this work is also to revive the concept of polygonal craters in the impact cratering community. The origin of polygonal craters: A fractured target material is a prerequisite for the formation of polygonal craters. Polygonal simple craters result when the excavation of the crater progresses more easily along a fracture (or some other plane of weakness) than in other directions [4]. This typically leads to a squarish outline, with crater rims making approximately an angle of 45° with the fracture directions [4,5]. However, experiments have shown that two perpendicular fracture directions can also lead to three rim orientations, i.e. a hexagonal crater [6]. Therefore the information gained from the study of small polygonal crater rims can not be unambiguously transferred to regional fracture directions. Polygonal complex craters are the result of slumping in the modification stage of the cratering process: the collapse of the rim takes place along some plane of weakness in the target [4]. Thus, the dominant fracture directions in the area can be directly measured from the orientations of the straight rim segments in complex polygonal craters. Methods: We have mapped the polygonal impact craters in the geologically diverse greater Hellas region (lat. 2°N-66°S, long. 8°W-208°W, Fig. 1) by using the Viking Orbiter Mars Digital Image Mosaic (MDIM). The resolution of the images is 0,231 km per pixel at the equator, but gets worse further south. The study area was divided into 40 blocks, each of which was examined independently by two researchers. We defined " polygonal crater " as being a crater with at least two straight rim segments and a clearly discernible angle between them. We disregarded craters where the polygonal outline was distinctively a result of post-impact processes. The strikes of the rim directions were measured from three major parts of the study area: the highland area south from Isidis (reaching the volcanic plains of Elysium Planitia in the northeastern corner of our study area) including parts of Hesperia Planum, Hellespontus Montes west from Hellas and the volcanic …