We determine the proportions of two mixed crater populations distinguishable by size distributions on the Moon .
A ” multiple power-law ” model is built to formulate crater size distribution N ( D ) \propto D ^ { - \alpha } whose slope \alpha varies with crater diameter D .
Fitted size distribution of lunar highland craters characterized by \alpha = 1.17 \pm 0.04 , 1.88 \pm 0.07 , 3.17 \pm 0.10 and 1.40 \pm 0.15 for consecutive D intervals divided by 49 , 120 and 251 km and that of lunar Class 1 craters with a single slope \alpha = 1.96 \pm 0.14 , are taken as Population 1 and 2 crater size distribution respectively , whose sum is then fitted to the size distribution of global lunar craters with D between 10 and 100 km .
Estimated crater densities of Population 1 and 2 are 44 \times 10 ^ { -5 } and 5 \times 10 ^ { -5 } km ^ { -2 } respectively , leading to the proportion of the latter 10 \% .
The results underlines the need for considering the Population 1 craters and the relevant impactors , the primordial main-belt asteroids which dominated the late heavy bombardment .