Venusian impact crater size‐frequency distributions, locations, and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism. An atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km, the size‐frequency distribution is close to the atmosphere‐free case. With this result, and assuming that the surface records a crater production population (a catastrophic resurfacing model, CRM), an age of cessation of rapid resurfacing of ∼ 500 Ma is obtained. Crater locations are widely dispersed across Venus and the hypothesis that they are completely spatially random (CSR) cannot be rejected. However, craters that show embayment by plains materials or modification by throughgoing faults (i.e., tectonized) are preferentially found in areas with relatively few craters overall. The primary region where these modified craters are found is the Aphrodite volcanotectonic zone, extending from Ovda Regio on the west to the region east of Atla Regio. These results, together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy, imply that a range of surface ages are recorded by the impact crater population; e.g., the Aphrodite zone is relatively young. An end‐member model (equilibrium resurfacing model, ERM) was developed to quantify resurfacing scenarios. In the ERM, Venus has been resurfacing at an average rate of approximately 1 km2 yr−1. However, the CRM and ERM are idealized end‐member representations of possible resurfacing histories. For both models, the resurfacing rate can be expressed as the product of resurfacing patch area a (normalized by planetary surface area) and the frequency ω of resurfacing events. Numerical simulations of resurfacing showed that there are two solution branches that satisfy the CSR constraint: a < 0.0003 (4° diameter circle ) and a > 0.1 (74° diameter circle). The former range corresponds to resurfacing diameters smaller than the average intercrater distance, whereas the latter is associated with large, infrequent events, resurfacing 10% of the planet every 50 Ma to 100% every 500 Ma. The observed fraction of embayed and tectonized craters further constrains values of a and only values near 0.0003 are admissible. The resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs, in any given geological episode, in only a limited number of regions on the planet.

Page 5 likely to yield the most reliable results. Kirchoff et al. (2011) provide a more recent comparison with three researchers (two expert, one novice without crater counting experience) from the same lab who used the same technique to identify, measure, and, in this case, classify craters by preservation state. They used Lunar Reconnaissance Orbiter Camera Wide-Angle Camera (LROC WAC) images of Mare Orientale. The two experienced analysts had counts that differed by 20-40% in a given diameter range, while the novice counter identified numerous features that are probably not craters, differing from the other two by >100% over some diameter ranges. They also had significant variation among the preservation states attributed to each crater, despite a relatively coarse fourpoint scale. This work showed that despite common thinking that crater counting is fairly easy and straightforward, there is a learning curve and an individual's crater counts should be discarded during the learning process. It also showed that even well defined crater morphologies may be difficult to classify uniformly. Hiesinger et al. (2012) also focused on lunar craters, in their case using LROC Narrow-Angle Camera (NAC) images at approximately 0.5 m/px. They were interested in reproducible results for better understanding the lunar cratering flux and performed a single test with two experienced researchers who used the same technique on the same image. The Heisinger et al.(2012) team found an overall variation of only ±2% between their analysts, a dispersion significantly less than previous work.What this brief review indicates is that while there has been some discussion in the literature about agreement between different researchers' crater identifications, (a) there has been no thorough discussion on researcher variability, (b) no published study discusses the variability when using different techniques for crater identification and measurement, (c) variation in crater morphology has not been discussed (e.g., sub-km craters appear substantially different at NAC pixel scales when compared with multi-km craters at WAC pixels scales), and (d) expert results have not been extensively compared with how well untrained or minimally trained crater counters do with the identification and measurement process. Given the proliferation of internet

[1] We present three investigations that use the Venusian impact crater population to constrain the planet's resurfacing history. We evaluate stereo-derived topography for 91 Venusian craters that have a diameter (D) greater than 15 km. Craters with radar-bright floors have greater rim-floor depths and rim heights than craters with radar-dark floors. . For a 60 km crater, this represents differences of 290 m in rim-floor depth and 240 m in rim height. We interpret these results to indicate that dark-floored craters have experienced postimpact volcanic embayment and filling. We examine the population of craters with D > 20 km that have radar-dark halos surrounding their continuous ejecta (114 craters). We find that a portion of the halo has been removed for almost all dark-floored craters, consistent with our interpretation that dark-floored craters have been affected by postimpact volcanism. Finally, we assessed geologic histories of 12 large impact structures with stereo coverage. All but one of these structures has experienced postimpact volcanism or tectonic deformation, often in multiple episodes. In summary, widespread volcanic and tectonic activity occurred throughout the time period of emplacement of the crater population. Postulated resurfacing histories that consider the majority of craters to be at the top of the stratigraphic column are invalid, and the mean surface age of Venus is young (∼150 My).Citation: Herrick, R. R., and M. E. Rumpf (2011), Postimpact modification by volcanic or tectonic processes as the rule, not the exception, for Venusian craters,

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inverse gravity trend observed for the terrestrial planets, and they are --50% deeper than current estimates for complex craters on the Earth. Unlike the other terrestrial planets, neither terrain-floor depths nor central structure heights increase with increasing crater diameter. An interesting trend for which we have no explanation is that on Venus, the Moon, Mars, and Ganymede, central peaks generally rise to within a constant elevation relative to the surrounding terrain, but that elevation is lower on the Moon and Mars than on Venus and Ganymede.