Abstract
Late accretion describes the final addition of Earth’s mass following Moon formation and includes a period of Late Heavy Bombardment (LHB), which occurred either as a short-lived cataclysm triggered by a late giant planet orbital instability or a declining bombardment during late accretion. Using genetically characteristic ruthenium and molybdenum isotope compositions of lunar impact–derived rocks, we show that the impactors during the LHB and the entire period of late accretion were the same type of bodies and that they originated in the terrestrial planet region. Because a cataclysmic LHB would have, in part, resulted in compositionally distinct projectiles, we conclude that the LHB reflects the tail end of accretion. This implies that the giant planet orbital instability occurred during the main phase of planet formation. Last, because of their inner solar system origin, late-accreted bodies cannot be the primary source of Earth’s water.