Change in the Earth–Moon impactor population at about 3.5 billion years ago

Minggang Xie, Zhiyong Xiao, Luyuan Xu, Wenzhe Fa & Aoao Xu

Nature Astronomy (2020), Published: 09 November 2020


“The bombardment of impactors (leftover planetesimals, asteroids and comets) created numerous impact craters on the Moon. The giant planets in the outer Solar System are believed to have experienced a dynamical instability, in which the migration of giant planets delivers impactors to the inner Solar System bodies. The difference between the population of large (diameter more than ~5 km) impact craters observed on heavily cratered lunar highlands and that on the lunar maria was thought to support the lunar Late Heavy Bombardment, which started ~0.6 billion years after planet formation and could have been caused by the late instability of giant planets. However, large craters on various-aged lunar surfaces have similar size–frequency distributions when considering the preferential erasure of small craters. In addition, dynamical and geochemical evidence favour an early instability of giant planets at ~4.5 Gyr ago. Here, we report the evolution at geological scales of regolith thickness on the Moon, which is a proxy for the change of the size–frequency distribution slope for Earth–Moon impactors with diameters less than ~50 m (which generate craters with diameters less than ~1 km (ref. 10)). We found an abnormally slow growth of regolith thickness per unit of impact flux before 3.5+0.3−0.6 Gyr ago (3σ uncertainty), which can best be explained by a population of craters of less than ~1 km whose size–frequency distribution had a shallower power-law slope (−2.57+0.30−0.16) than that afterward (−3.24+0.03−0.06). The transition time at ~3.5 Gyr ago supports the early instability of giant planets, in which dominant Earth–Moon impactors changed from leftover planetesimals to asteroids5. The value of −3.24 is consistent with the preferential delivery of small asteroids via Yarkovsky–YORP effects.”