The preliminary analysis of polygonal impact craters within greater Hellas region, Mars

The polygonal planimetric shape of impact craters has been known for a long time, but has not been discussed much in the past. Polygonal craters exist on all kinds of celestial bodies that have fractured rigid crusts. They are also found on the Earth (e.g., Meteor Crater, Soderfjarden). Polygonal craters are thought to have been formed by two possible mechanisms. Simple polygonal craters, such as the square-shaped Meteor Crater, result when the excavation flow opens the crater, tearing the target more easily along pre-existing fractures or other planes of weakness. Complex polygonal craters are supposed to have formed during the modification stage, when rocks of the crater rim slump along the fractures in the target. Both mechanisms lead to straight segments of the rim. However, in simple craters the straight rim is typically at some angle, usually about 45 degrees, to the direction of the fractures, whereas in complex craters the rims are thought to be parallel to them. Thus, the regional fracture trends are easier to deduce from complex polygonal craters. We have studied the distribution and rim orientations of polygonal craters within the geologically versatile greater Hellas region in the southern hemisphere of Mars. Our results (rose diagrams of straight rim segments) indicate the existence and dimensions of radial and concentric fracture patterns around the impact basins of Hellas and Isidis. Such deformation patterns have been observed previously using other indicators of tectonism (grabens, etc.). Other major causes of fracture directions visible in our rose diagrams are the volcanoes in or near the study area. Especially Elysium Mons has created a prominent radial fracture system. Our results correlate well with those obtained by other methods, thus indicating that the use of polygonal craters in finding, studying, and mapping structural properties of a planet's crust is justified.