Suggestion that recent (≤ 3 Ga) flux of kilometer and larger impactors in the Earth-Moon system has not been constant

Abstract

Lunar impact crater chronologies have been developed by combining carefully measured crater densities of lunar terrains with radiometric ages provided by returned samples. However, due to the sparse coverage of the samples during the last three billion years, this part of the chronologies is not well constrained. Lunar crater chronologies generally assume that the bombardment rate has been relatively constant during this epoch for all impactor sizes. Nevertheless, evidence has been gathering that this may not be the case for impactors larger than several hundred meters; however, there may be biases related to some of this evidence. The break-up of large asteroids in the Main Asteroid Belt to make asteroid families could be a source of a changing impact flux in the Earth-Moon system, especially when the families form near strong orbital resonances with the gas giants. In order to further explore the state of the impact flux from today to three billion years ago for impactors larger than ~5 km, we calculate crater retention model ages of 43 lunar craters 50 km and larger in diameter (D). Selected craters were initially suggested by United States Geological Survey geological maps of the Moon and Wilhems (1987) to have formed during the Copernican and Eratosthenian. We use the density of small (D < a few km) craters superposed on their floors, along with Model Production Function (MPF) lunar chronology (Marchi et al., 2009). For this purpose, we assume that the smaller impactors forming the superposed craters follow a constant flux as indicated by the MPF and supported by models of the dynamical evolution of the impactor population. We use the model ages as a proxy for the impact flux of larger impactors and test whether their distribution in time is consistent with a constant flux using two statistical analyses. Our results suggest that the flux of these larger impactors could be variable during the last three billion years, with hints of a relative increase in flux occurring ~2 billion years ago and a decrease in flux ~1 billion years ago. Thus, our results support the evidence that the impact flux has varied on the Moon for impactors over a few kilometers in diameter.