A new chronology from debiased crater densities: Implications for the origin and evolution of lunar impactors

Abstract

The giant planets are believed to experience dynamic instability, delivering numerous impactors (asteroids, comets and leftover planetesimals) to the Moon. Dynamic simulations show that the impacts of leftover planetesimals decay much quicker than those from the main belt. Due to the relatively slow decay of early lunar impact flux directly inferred from observed crater densities, the dominant impactors were believed to have always been the main belt asteroids instead of rapidly decreasing leftover planetesimals. However, previously-derived densities are biased, with topographic degradation thought to increase the apparent density over time due to degradation-induced enlargement of crater diameter, coeval targets with various properties exhibiting different crater densities, and preferential erasure of small craters by the formation of larger ones. Here we establish a model with consideration of all these effects to derive crater densities on a competent-rock-like reference target (denoted as debiased crater densities), showing a constant flux prolonged back to ∼3.8 billion years ago (Ga) and a more intense earlier impact flux by a factor up to 100 compared with the standard production and chronology functions. We find that ancient (older than ∼3.8 Ga) craters were dominantly produced by leftover planetesimals rather than asteroids, consistent with the early migration of the giant planets.