Formation of Lunar Basins from Impacts of Leftover Planetesimals

TL;DR

This study suggests that lunar basins primarily formed from impacts of leftover planetesimals from the inner Solar System, challenging previous asteroid-based impact models and explaining the early impact record.

Contribution

It introduces a new model indicating leftover planetesimals caused most lunar basin formations, altering the understanding of early Solar System impact history.

Findings

Most impactors were leftover planetesimals at 0.5-1.5 au.

The number of impacts exceeds observed lunar basins, implying early impacts are unrecorded.

Imbrium basin formation has a 15-35% probability in the model.

Abstract

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 our 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.