Martian impact craters and emplacement of ejecta by surface flow

Several types of Martian impact craters have been recognized. The most common type, the rampart crater, is distinctively different from lunar and Mercurian craters. It is typically surrounded by several layers of ejecta, each having a low ridge or escarpment at its outer edge. Outward flow of ejecta along the ground after ballistic deposition is suggested by flow lines around obstacles, the absence of ejecta on top and on the lee side of obstacles, and the large radial distance to which continuous ejecta is found. The peculiar flow characteristics of the ejecta around these craters are tentatively attributed to entrained gases or to contained water, either liquid or vapor, in the ejecta as a result of impact melting of ground ice. Ejecta of other craters lacks flow features but has a marked radial pattern; ejecta of still other craters has patterns that resemble those around lunar and Mercurian craters. The internal features of Martian craters, in general, resemble their lunar and Mercurian counterparts except that the transition from bowl shaped to flat floored takes place at about 5-km diameter, a smaller size than is true for Mercury or the moon.