The Impact Angles of Different Crater Forms on Mars

Introduction: Previous surveys have been conducted of the Venusian and lunar impact crater populations in an effort to empirically determine the onset angle of various oblique impact forms. Those surveys showed a general consistency with predictions from small-scale experiments [1,2,3]. That previous work also showed that in the presence of a dense atmosphere the onset angle increases for various phenomena, and ejecta is carried downrange in a turbulent cloud. A weakness of the previous surveys was the small numbers of craters sampled. Venus has a young surface and the dense atmosphere prevents small craters from forming. Space weathering and gardening on the mooon make it difficult to discern the distal ejecta for all but the most recent impacts. On Mars there are hundreds of well-imaged craters located on flat surfaces, and ejecta blankets are easy to distinguish because they are emplaced as ramparts. We have surveyed Martian craters over a large size range to empirically determine the angles for which various phenomena occur. Procedure: Using Viking imagery we surveyed a total of 466 impact craters in the northern plains of Mars from 1 – 128 km in diameter. Craters over 5 km were analyzed using the global MDIM at a resolution of 231 m/pixel, and the smaller craters were analyzed using the high-resolution MDIMs at 58 m /pixel. Five distinct ejecta planform types were observed. Assuming that increasing ejecta asymmetry indicates decreasing impact angle, the categories in order of decreasing impact angle are as follows: • Symmetric craters have circular crater shapes and no obvious impact direction. The ejecta blanket is equal in size and shape in all directions. • Offset craters have circular crater shapes. They have asymmetric ejecta blankets that have either a lack of (but not absence of) ejecta in the uprange direction or a surplus of ejecta in the downrange direction. The crater rim is not centered in the ejecta blanket, it is offset in the uprange direction. • Forbidden Zone craters have circular crater shapes. These craters have asymmetric ejecta blankets with an absence of ejecta in the uprange impact direction, often in the shape of a V. They may or may not have a surplus of ejecta in the downrange impact direction. • Crossrange Lobe craters can have either circular craters or craters that are slightly elongate in the impact direction. Ejecta material is mainly concentrated perpendicular to the impact direction in two crossrange lobes. Small amounts of ejecta still appear uprange and downrange of the crater. • Butterfly craters are often, but not always, elongate in the impact direction. Two distinct forbidden zones appear uprange and downrange of the crater. All of the ejecta is located perpendicular to the impact direction in a "butterfly wing" pattern.