Constraints on planetesimal disk mass from the cratering record and equatorial ridge on Iapetus

TL;DR

This study uses crater counts and ridge morphology on Iapetus to estimate the total impact bombardment it experienced, constraining the early Solar System's planetesimal disk mass and challenging existing models.

Contribution

It provides new constraints on the planetesimal disk mass and early Solar System bombardment levels based on Iapetus's surface features and crater record.

Findings

Iapetus received less than 20% of the predicted bombardment by the classic Nice model.

The observed features suggest a planetesimal disk mass of approximately 12-34 Earth masses.

The results challenge traditional models of Solar System evolution and impact history.

Abstract

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 $M_{D}\sim12-34M_{\oplus}$.