Formation of Lunar Basins from Impacts of Leftover Planetesimals

Abstract The Moon holds important clues to the early history of the Solar System. The large scars evident on its near side are impact basins filled with dark basalt; some 50 lunar basins (crater diameter $D>300$ km) have been recognized overall$^1$. The question of the origin of basin-forming impactors remained unsettled. Here we show that most impactors were rocky planetesimals left behind at $\sim 0.5$--$1.5$ 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 $\sim200$ Myr of impacts is not recorded on the lunar surface$^{2,3}$. The Imbrium basin formation (age$^4$ $\simeq 3.92$~Gyr; impactor diameter$^{5,6}$ $d>100$ km) occurs with a 15--35\% probability in the model. Imbrium must have formed unusually late to have only two smaller basins (Orientale and Schr\"odinger) forming afterwards. The model predicts $\simeq 20$ $d>10$-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$^7$.