A possible impact lull in the middle of the Eratosthenian period suggested by the Chang’e-5 and 6 missions

Context. It was generally believed that lunar impact flux over the past 3.5 billion years has been almost constant. However, there are still considerable age gaps, causing the evolution of impact flux to remain elusive. Recently, the Chang’e-5 and 6 missions collected samples from the Moon, and the respective ages of the mare basalt units containing the landing sites were dated to be 2.0 and 2.8 billion years ago (Ga). Aims. With this work, we aim to investigate the constancy of the lunar impact flux. Methods. We measured the densities of craters in both Chang’e-5 and 6 landing mare basalt units. Results. We find that the density of craters with diameters ranging from about 200 m to 1 km in the Chang’e-6 unit is almost identical to that of the Chang’e-5 unit. The density similarity suggests an impact lull (relatively few impacts on average) between 2.0 and 2.8 Ga. Consequently, the population of near-Earth objects (NEOs) is not in a quasi-steady state, with this being supported by a spike found at about 2.8-3.4 Ga. The results suggest that large, rare asteroid family-forming events in the main asteroid belt, rather than background main belt asteroids, are the main (>90 ± 2%; 1σ uncertainty) source of subhectometer NEOs. Alternatively, to be compatible with a constant flux, the strength of the Chang’e-5 mare basalt has to be about 30% lower than that of Chang’e-6.