The era in which this activity occurred, and whether or not it is continuing, is unknown. The Viking orbiter pictures revealed some unique characteristics of Martian impact craters. The ejecta pattern around most fresh Martian impact craters is distinctively different from that around lunar and mercurian craters. On the Moon and on Mercury, the ejecta typically have a coarse, disordered texture close to the rim. Farther out, the texture becomes finer and, with increasing radial distance, grades imperceptibly into dense fields of secondary craters and rays.
Most Martian craters have a quite different ejecta pattern.
The ejecta commonly appear to consist of several layers, with the outer edge of each marked by a low ridge or escarpment. Features on the ejecta surfaces include closely spaced radial striae and concentric grooves, ridges, and scarps, especially toward the outer margin. These unique Martian features were seen vaguely in the Mariner 9 pictures and tentatively attributed to wind action. Viking pictures show that many of the peculiar characteristics of Martian craters are primary emplacement features not due to wind. The fresher the crater appears, the better preserved are the striae, ramparts, and concentric features.