Highly siderophile elements in Earthâ(tm) s mantle as a clock for the Moon-forming impact

According to the generally accepted scenario, the last giant impact on Earth formed the Moon and initiated the final phase of core formation by melting Earthâ (tm) s mantle. A key goal of geochemistry is to date this event, but different ages have been proposed. Some argue for an early Moon-forming event, approximately 30 million years (Myr) after the condensation of the first solids in the Solar System, whereas others claim a date later than 50â Myr (and possibly as late as around 100â Myr) after condensation. Here we show that a Moon-forming event at 40â Myr after condensation, or earlier, is ruled out at a 99.9 per cent confidence level. We use a large number of N-body simulations to demonstrate a relationship between the time of the last giant impact on an Earth-like planet and the amount of mass subsequently added during the era known as Late Accretion. As the last giant impact is delayed, the late-accreted mass decreases in a predictable fashion. This relationship exists within both the classical scenario and the Grand Tack scenario of terrestrial planet formation, and holds across a wide range of disk conditions. The concentration of highly siderophile elements (HSEs) in Earthâ (tm) s mantle constrains the mass of chondritic material added to Earth during Late Accretion. Using HSE abundance measurements, we determine a Moon-formation age of 95â ±â 32â Myr after condensation. The possibility exists that some late projectiles were differentiated and left an incomplete HSE record in Earthâ (tm) s mantle. Even in this case, various isotopic constraints strongly suggest that the late-accreted mass did not exceed 1 per cent of Earthâ (tm) s mass, and so the HSE clock still robustly limits the timing of the Moon-forming event to significantly later than 40â Myr after condensation.