Abstrakt
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 similar to 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 similar to 200 Myr of impacts are not recorded on the lunar surface. The Imbrium basin formation (age similar or equal to 3.92 Gyr; impactor diameter d greater than or similar to 100 km) occurs with a 15%-35% probability in our model.
Imbrium must have formed unusually late to have only two smaller basins (Orientale and Schrodinger) forming afterwards. The model predicts similar or equal to 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.