  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.

Earth and Planetary Astrophysics