The surface of Mars 1. Cratered terrains

Mariner 6 and 7 pictures show that craters are the dominant landform on Mars and that their occurrence is not correlated uniquely with latitude, elevation, or albedo markings. Two distinct morphological classes are recognized: small bowl-shaped and large flat-bottomed. The former show little evidence of modifications, whereas the latter appear generally more modified than lunar upland craters of comparable size. A regional maria/uplands dichotomy like the moon has not yet been recognized on Mars. Crater modification on Mars has involved much greater horizontal redistribution of material than in the lunar uplands. It is possible that there are erosional processes only infrequently active. Analysis of the natures and fluxes of bodies that have probably impacted the moon and Mars leads to the likelihood that most of the large flat-bottomed craters on Mars have survived from the final phases of planetary accretion. Significant crater modification, however, has taken place more recently on Mars. Inasmuch as the present small bowl-shaped craters evidence little modification, the postaccretion crater-modification process on Mars may have been primarily episodic rather than continuous. The size-frequency distribution of impacting bodies that produced the present small Martian bowl-shaped craters differs from that responsible for post-mare primary impacts on the moon by a marked deficiency of large bodies. Survival of crater topography from the end of planetary accretion would make any hypothetical earthlike phase with primitive oceans there unlikely. The traditional view of Mars as an earthlike planetary neighbor in terms of its surface history is not supported by the picture data.