  Iapetus, the outermost regular satellite of Saturn, has a drastic albedo
dichotomy and an equatorial circumferential ridge that reaches heights of 20 km
and widths of 70 km. This moon is thought to have formed concurrently with
Saturn, and so would have experienced an intense bombardment after its
formation. The ridge, which has been inferred to be one of the most ancient
features on Iapetus' surface, could reasonably be expected to have been eroded
by impacts; however, it has retained long continuous sections and a nearly
pristine triangular shape with ridge slopes reaching $\sim$ 40$^{\circ}$. We
use these observations, along with crater counts on Iapetus' surface, to
constrain the total bombardment mass experienced by the satellite since its
formation. The ridge morphology and the crater population recorded on Iapetus
indicate it received less than 20% of the bombardment predicted by the classic
Nice model for early Solar System evolution. Under the recently proposed
scenarios of planetsimal-driven migration of the young outer planets including
more realistic disk conditions, our results would imply a planetesimal disk
mass of $M_{D}\sim12-34M_{\oplus}$.
