The lunar cratering record provides valuable information about the late accretion history of the inner solar system. However, our understanding of the origin, rate, and timing of the impacting projectiles is far from complete. To learn more about these projectiles, we can examine crater size‐frequency distributions (CSFDs) on the Moon. Here we reinvestigate the crater populations of 30 lunar basins (≥ 300 km) using the buffered nonsparseness correction technique, which takes crater obliteration into account, thus providing more accurate measurements for the frequencies of smaller crater sizes. Moreover, we revisit the stratigraphic relationships of basins based on N(20) crater frequencies, absolute model ages, and observation data. The buffered nonsparseness correction‐corrected CSFDs of individual basins, particularly at smaller crater diameters are shifted upward. Contrary to previous studies, the shapes of the summed CSFDs of Pre‐Nectarian (excluding South Pole‐Aitken Basin), Nectarian (including Nectaris), and Imbrian (including Imbrium) basins show no statistically significant differences and thus provide no evidence for a change of impactor population.

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 higher N(300) crater density on Mercury than on the Moon, but similar N(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 on N(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.

The analysis of crater size‐frequency distributions (CSFDs) is a widely used technique to date and investigate planetary surface processes. There are two well‐established crater measurement techniques, traditional crater counting and buffered crater counting, and two new geometric corrections, nonsparseness correction and buffered nonsparseness correction. The new techniques consider the effects of crater obliteration and subsequent recratering while measuring CSFDs in areas of high crater density. Currently, the ArcGIS add‐in CraterTools can be used to apply the well‐established techniques. The tool relies on Esri's ArcGIS environment and is restricted to 32 bit and single‐core computing. These limitations make the implementation of the new geometric corrections in CraterTools inefficient, as the new techniques are computationally more intensive than the well‐established ones. To this end, we developed CSFD Tools, an application to conduct CSFD measurements from shapefiles. It supports 64 bit and multicore data processing and uses existing open geospatial libraries. Open libraries, however, conduct spatial measurements on a Cartesian plane and do not take a curved planetary surface into account. Therefore, we implemented methods for geodesic measurements and workarounds for the geodesic modification of polygon data to minimize map distortion effects during CSFD measurements. As a result, the new nonsparseness correction and buffered nonsparseness correction techniques can be applied through a software tool.

The crater record of a planetary surface unit is often analyzed by its cumulative size-frequency distribution (CSFD). Measuring CSFDs involves traditional approaches, such as traditional crater counting (TCC) and buffered crater counting (BCC), as well as geometric corrections, such as nonsparseness correction (NSC) and buffered nonsparseness correction (BNSC). NSC and BNSC consider the effects of geometric crater obliteration on the CSFD. On the Moon, crater obliteration leads to two distinct states in which obtained CSFDs do not match the production CSFD-crater equilibrium and nonsparseness. Crater equilibrium occurs when each new impact erases a preexisting crater of the same size. It is clearly observed on lunar terrains dominated by small simple craters with steep-sloped production CSFDs, such as Imbrian to Eratosthenian-era mare units. Nonsparseness, on the other hand, is caused by the geometric overlap of preexisting craters by a new impact, which is also known as "cookie cutting." Cookie cutting is most clearly observed on lunar terrains dominated by large craters with shallow-sloped production CSFDs, such as the pre-Nectarian lunar highlands. We use the Cratered Terrain Evolution Model (CTEM) to simulate the evolution of a pre-Nectarian surface unit. The model was previously used to simulate the diffusion-induced equilibrium for small craters of the lunar maria. We find that relative to their size, large craters contribute less to the diffusion of the surrounding landscape than small craters. Thus, a simple scale dependence cannot account for the per-crater contribution to degradation by small simple and large complex craters.

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