Formation of Lunar Basins from Impacts of Leftover Planetesimals
Résumé
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 are 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 our model. Imbrium must have formed unusually late to have only two smaller basins (Orientale and Schrödinger) 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.
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