Contribution of multi-ring basins to lunar production functions

Crater size-frequency distribution (CSFD) measurements provide a valuable tool for dating surfaces on terrestrial planetary bodies. In order to derive an age from CSFD measurements, a production function (PF) is required to reflect the size-frequency distribution of craters being formed. However, the most commonly used PFs are only valid to crater diameters up to 300 km. An expansion of the PFs would be beneficial to improve the understanding of the early bombardment history of the Moon. However, there are two main issues affecting the interpretation of the CSFDs of large and multi-ring basins: the determination of a standard main basin rim for multi-ring basins and the low number of large basins. Previously, numerous attempts have been made to define the representative diameters for (multi-ring) basins, which appear in many lunar catalogs. The comparison of four such lunar crater catalogs revealed significant differences in defining basin boundaries and diameters. Therefore, we created a new crater catalog for lunar craters larger than 100 km based on topographic and gravity data and using a consistent definition for basin diameters. We found that on the highlands, the CSFD measurement has not reached equilibrium. Given that the number of empirically measured basins is limited, this investigation naturally must deal with low-number statistics. However, the available data for basins >300 km still allow an empirical analysis to provide insights into this poorly understood part of the lunar PF. We find that the shape of the CSFD is consistent with the impactor size-frequency distribution (SFD) of the Main Asteroid Belt, suggesting an origin from the Main Asteroid Belt. The presence of an additional impactor population cannot be observed in our data, although we cannot exclude the possibility of a second population with a similar SFD. Using the highland CSFD measurement as a reference, we found interesting large discrepancies between the measured and the calculated number of craters and basins on the lunar mare. For instance, we measured 30 craters and basins ≥100 km on mare, but expected 79 using the PF of Neukum et al. (2001). In addition, we identified 9 basins ≥300 km on mare areas, but expected only 5 basins when applying the PF of Neukum et al. (2001).