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.

We conducted a detailed photogeological analysis of the northern portion of the South Pole‐Aitken (SPA) basin (10–60°S, 125–175°W) and compiled a geological map (1:500,000 scale) of this region. Our new absolute model age for the Apollo basin, 3.98 + 0.04/−0.06 Ga, provides a lower age limit for the formation of the SPA basin. Some of the plains units in the study area were formed by distal ejecta from remote craters and basins. The characteristic concentrations of FeO and TiO2 of other plains are indicative of their volcanic origin. The oldest volcanic materials occur near the center of the SPA basin and have an Early Imbrian age of ~3.80 + 0.02/−0.02 Ga. Late Imbrian volcanic activity occurred in and around the Apollo basin. In total, the volcanic plains cover ~8% of the map area and cannot account for the extensive SPA iron signature. The sources of the signature are the oldest materials on the SPA floor (FeO ~11–14.5 wt%). In contrast, the ejecta composing the SPA rim are significantly poorer in FeO (<7.5 wt%). The signature could be related to the differentiation of the SPA impact melt. However, the spatial segregation of the ancient iron‐rich and iron‐poor materials suggests that the SPA iron signature predated the basin. Thus, the signature might be explained by a pre‐SPA lunar crust that was stratified with respect to the iron concentrations, so that the SPA impact excavated the upper, iron‐poorer portion of the crust to form the SPA rim and exposed the deeper, iron‐richer portion on the floor of the basin.

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.