Modeling the chronologies and size distributions of Ceres and Vesta craters

Abstract

We infer the crater chronologies of Ceres and Vesta from a self-consistent dynamical model of asteroid impactors. The model accounts for planetary migration/instability early in the solar system history and tracks asteroid orbits over 4.56 Gy. It is calibrated on the current population of the asteroid belt. The model provides the number of asteroid impacts on different worlds at any time throughout the solar system history. We combine the results with an impactor-crater scaling relationship to determine the crater distribution of Ceres and Vesta and compare these theoretical predictions with observations. We find that: (i) The Ceres and Vesta chronologies are similar, whereas they significantly differ from the lunar chronology. Therefore, using the lunar chronology for main belt asteroids, as often done in previous publications, is incorrect. (ii) The model results match the number and size distribution of large (diameter $>90$ km) craters observed on Vesta, but overestimate the number of large craters on Ceres. This implies that large crater erasure is required for Ceres. (iii) In a model where planetary migration/instability happens early, the probability to form the Rheasilvia basin on Vesta during the last 1 Gy is 10\%, a factor of $\sim1.5$ higher than for the late instability case and $\sim2.5$ times higher than found in previous studies. Thus, while the formation of the Rheasilvia at $\sim1$ Gy ago (Ga) would be somewhat unusual, it cannot be ruled out at more than $\simeq1.5\sigma$. In broader context, our work provides a self-consistent framework for modeling asteroid crater records.