Asymmetrical distribution of 1–20 km craters on the Moon

Previous studies have provided evidence for the synchronous rotation induced cratering asymmetry on lunar surface through numerical simulations and statistical analysis of a limited number of fresh craters. In this study, we reevaluated cratering asymmetry in lunar highland from (70°W, 60°N) to (70°E, 60°S) region using a new crater catalogue with diameters (D) ranging from 1 to 20 km. By utilizing a depth-to-diameter (d/D) ratio constraint to exclude the interference of degraded and secondary craters, we observed significant asymmetry in craters with d/D > 0.15. Moreover, leveraging the characteristic that larger diameter craters (D > 7 km) are less susceptible to degradation, we observed a more pronounced asymmetry with increasing diameter. Particularly, impact craters with larger D and d/D ratios (D > 7 km, d/D > 0.15) displayed an asymmetrical longitudinal distribution, aligning with predictions from the theoretical model. In the diameter range of 10 km–20km, for craters with d/D > 0.15, we observed that new crater influx occurring after 4.0 Ga years ago contributed little to this particular crater population. Therefore, we suggest that the cratering asymmetry was already present before 4.0 Ga. Due to the non-uniform ejecta from the Orientale Basin onto the highland regions, a significant number of smaller impact craters (1–5 km) have degraded or disappeared in the leading region, thereby diminishing the manifestation of the cratering asymmetry. The pronounced asymmetry exhibited in our statistical results might suggest the existence of a significant population of low-velocity impactors in early impact period (>4Ga) around the cis-lunar space.