  Impact basins identified by Mariner 10 and Messenger flyby images provide us
a fossilized record of the impactor flux of asteroids on Mercury during the
last stages of the early Solar System. The distribution of these basins is not
uniform across the surface, and is consistent with a primordial synchronous
rotation (Wieczorek et al. 2012). By analyzing the size of the impacts, we show
that the distribution for asteroid diameters D < 110 km is compatible with an
index power law of 1.2, a value that matches the predicted primordial
distribution of the main-belt. We then derive a simple collisional model
coherent with the observations, and when combining it with the secular
evolution of the spin of Mercury, we are able to reproduce the present 3/2
spin-orbit resonance (about 50% of chances), as well as a primordial
synchronous rotation. This result is robust with respect to variations in the
dissipation and collisional models, or in the initial spin state of the planet.
