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

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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 bodies1,2. 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 maria3 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 planets4–6. However, large craters on various-aged lunar surfaces have similar size–frequency distributions when considering the preferential erasure of small craters7,8. In addition, dynamical and geochemical evidence favour an early instability of giant planets at ~4.5 Gyr ago2,5,9. 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 $$3.5_{ - 0.6}^{ + 0.3}$$ 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 ( $$- 2.57_{ - 0.16}^{ + 0.30}$$ ) than that afterward ( $$- 3.24_{ - 0.06}^{ + 0.03}$$ ). 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 effects11. The change in growth of the lunar regolith thickness around 3.5 Gyr ago, a consequence of a change in population of the impactor bodies from planetesimals to asteroids, indicates that the instability of giant planets happened early.