Initial Results from a Global Database of Mercurian Craters

Using MDIS images, we have compiled a database of all impact craters on Mercury with D > 5 km (36K total craters), excluding obvious secondaries (those in chains and clusters). For craters with D > 10 km (12K craters), we have characterized interior and exterior morphology, degradation state, and the presence of post-impact filling. The database represents a unique resource for understanding the resurfacing history of Mercury, examining regional variations in near-surface rheology, and studying the effects of target and impactor properties on final crater forms. We will present initial results on all of these topics. Here we highlight a couple of the significant results. We have used the database to plot the spatial densities of impact craters in different size ranges. Around several fresh peak ring basins over 150 km in diameter, such as Derain (D = 175 km, attached image), at ~4 crater radii from the rim we see an annulus of high spatial density of craters at 5-10 km in diameter that is not present in spatial density plots of craters 10-20 and 20-50 km in diameter. This confirms the presence of a previously hypothesized population of large (D > 5 km) distal secondaries. By virtue of its proximity to the Sun and a mix of asteroidal and cometary impactors, Mercury is expected to have a wide range of impactor velocities. We have been examining fresh craters in the 30-50 and 100-140 km diameter range and sorting them according to proximity to other similar-sized craters, with the idea that craters formed close to each other likely impacted similar targets. If impactor velocity is a significant factor in determining final crater appearance or depth, then there should be noticeable differences between nearby, same-sized craters. We have observed a few possible exceptions that warrant further examination, but generally we find that close-proximity crater pairs/clusters are identical in appearance and shape.