The Flux of Large Impacts on the Moon Enhanced near the Beginning of the Copernican Period

Abstract The size–frequency distribution (SFD) of lunar impactors was believed to be time independent for the last 3.5 billion years (Gyr). Here, I use linear and nonlinear topographic diffusion models to determine the degradation states (the integral of diffusivity over time) of 13,103 craters on the lunar maria, and find that the degradation rate (the degradation state per unit of crater production density), which should be constant if the impactor SFD is unchanged, varies with crater production density by a factor of 3.0–3.7. This crater density-dependent degradation rate results from the time-dependent SFD shape of lunar impactors, with the flux of large (greater than about 50 m in diameter) impactors increasing about 1.5–1.6 ± 0.1 billion years age (Ga). The enhanced flux potentially results from the formation of large asteroid families near resonances. This work emphasizes the importance of future missions to explore young surfaces to remove the age gap of lunar samples between 53 Ma and 2 Ga.