Formation of Lunar Basins from Impacts of Leftover Planetesimals

The Moon holds important clues to the early evolution of the Solar System. Some 50 impact basins (crater diameter D>300 km) have been recognized on the lunar surface, implying that the early impact flux was much higher than it is now. The basin-forming impactors were suspected to be asteroids released from an inner extension of the main belt (1.8-2.0 au). Here we show that most impactors were instead rocky planetesimals left behind at 0.5-1.5 au after the terrestrial planet accretion. The number of basins expected from impacts of leftover planetesimals largely exceeds the number of known lunar basins, suggesting that the first ~200 Myr of impacts is not recorded on the lunar surface. The Imbrium basin formation (age 3.92 Gyr, impactor diameter d>100 km) occurs with a 15-35% probability in the model. Imbrium must have formed unusually late to have only two smaller basins (Orientale and Schrodinger) forming afterwards. The model predicts ~20 d>10-km impacts on the Earth 2.5-3.5 Gyr ago (Ga), which is comparable to the number of known spherule beds in the late Archean.