On the craters of Mare Tyrrhenum (MC-22), Mars

2668 craters were identified and digitized from Mariner 9 imagery in U.S.G.S. quadrangle 'Mare Tyrrhehum' (MC-22), Mars in a study to supplement photogeologic mapping. A crater morphology classification, the X/Y machine plotting of each crater morphology type, and the isopleth analysis of crater size ranges resulted in the identification of 5 physiographic/stratigraphic units in close accordance with those of King (1974). Crater size frequency analysis of the stratigraphic unit crater densities and populations suggest: (a) a very old unit in the east of the quadrangle, showing giant (> 80 km. diameter) craters thought to date back to the last phases of planetary accretion (b) a western cratered unit with smaller densities and populations of craters than the east (c) a central smooth plains region characterized by populations of small craters (< 10 km. diameter), and modified from the cratered terrains by volcanic and aeolian processes. Nearest neighbor analyses suggest that small (< 10 km. diameter) craters of the western cratered terrains have been modified in morphology by a process which reverted them from Type 4 (fresh) to Type 1 (ghost). A lack of clustering of fresh craters from the smooth plains suggests a non volcanic origin for craters within 'A' frame limits of resolution. Concentric terraced and central peak craters indicate an ejecta blanket from the basin forming event of Hellas, across the south-western half of the quadrangle. Analysis of elliptical crater major axes suggests orientations concommitant with a regional modification from the above basin forming event. A population of bowl shaped craters (< 5 km. diameter) superimposes all stratigraphic units.