Re-examination of the Population, Stratigraphy, and Sequence of Mercurian Basins: Implications for Mercury's Early Impact History and Comparison With the Moon

Mercury has one of the best-preserved impact records in the inner solar system due to the absence of an atmosphere and relatively unmodified ancient surface. However, our knowledge of the early impact record and the nature of the impacting projectiles are far from complete. To get a better understanding of the early impact history, we examined large impact basins (D >= 300 km) on Mercury. Here we cataloged 94 basins, 80 of which we classify as certain or probable, 1.7 times more than previously recognized. We re-evaluate the crater densities of basins using the buffered nonsparseness correction technique, which we successfully applied for the Moon. In contrast with a previous study, we find that basins have a slightly higherN(300) crater density on Mercury than on the Moon, but similarN(500) basin densities. Based on these results and comparison with the Moon, we infer that no more than half of the basin record remains observable and basins older than Borealis have generally been erased from the basin record. Furthermore, we establish the stratigraphic relationships of basins based onN(25) crater frequencies, absolute model ages, and observations of crosscutting relationships. Similarly to our previous study on the Moon, we found no evidence for a change in the size-frequency distribution of the impacting population; thus, our results are consistent with a single impactor population that bombarded Mercury's surface.