Reconstructing the late-accretion history of the Moon

The importance of highly siderophile elements (HSEs; namely, gold, iridium, osmium, palladium, platinum, rhenium, rhodium and ruthenium) in tracking the late accretion stages of planetary formation has long been recognized. However, the precise nature of the Moon's accretional history remains enigmatic. There is a substantial mismatch in the HSE budgets of the Earth and the Moon, with the Earth seeming to have accreted disproportionally more HSEs than the Moon¹. Several scenarios have been proposed to explain this conundrum, including the delivery of HSEs to the Earth by a few big impactors¹, the accretion of pebble-sized objects on dynamically cold orbits that enhanced the Earth's gravitational focusing factor², and the `sawtooth' impact model, with its much reduced impact flux before about 4.10 billion years ago³. However, most of these models assume a high impactor-retention ratio (the fraction of impactor mass retained on the target) for the Moon. Here we perform a series of impact simulations to quantify the impactor-retention ratio, followed by a Monte Carlo procedure considering a monotonically decaying impact flux⁴, to compute the impactor mass accreted into the lunar crust and mantle over their histories. We find that the average impactor-retention ratio for the Moon's entire impact history is about three times lower than previously estimated^1,3. Our results indicate that, to match the HSE budgets of the lunar crust and mantle^5,6, the retention of HSEs should have started 4.35 billion years ago, when most of the lunar magma ocean was solidified^7,8. Mass accreted before this time must have lost its HSEs to the lunar core, presumably during lunar mantle crystallization⁹. The combination of a low impactor-retention ratio and a late retention of HSEs in the lunar mantle provides a realistic explanation for the apparent deficit of the Moon's late-accreted mass relative to that of the Earth.