Impact craters of Venus: A continuation of the analysis of data from the Venera 15 and 16 spacecraft

Additional studies of Venusian impact craters have been made based on an analysis of Venera 15/16 radar imagery and altimetry. The crater population on Venus has been subdivided into groups representing different morphological classes and types. The craters display the size-dependent variations in morphology which are well known from other planets. Assuming a crater production rate based on estimates by Hartmann et al. (1981), their areal density indicates an age for the total population of approximately 1 b.y. Alternative estimates based on the assumption that the recent cratering rates on the earth and Venus were similar would give a significantly lower age. The areal and size frequency distribution of a number of circular features of unclear origin have also been analyzed, and it is proposed that some of these features may be highly degraded and modified impact craters corresponding to a population approximately 3 b.y. in age. A suggested model of the effect of the atmosphere on the crater diameter-frequency curve indicates that cometary impacts played only a minor role in forming the contemporary Venus environment. A comparative study of craters in the area where the Arecibo earth based radar and Venera 15/16 images overlap indicates that the radar dark inner zones of the craters in the high incidence angle Arecibo images correspond to the flat bottoms of the craters seen in the Venera images.