Highly Siderophile Elements in the Earth's Mantle as a Clock for the Moon-forming Impact

TL;DR

This study uses highly siderophile element data and N-body simulations to constrain the timing of the Moon-forming impact, concluding it occurred at least 40 million years after Solar System formation, with a best estimate around 95 million years.

Contribution

It provides a robust geochemical and dynamical framework to date the Moon-forming impact, ruling out earlier dates and refining the impact timing estimate.

Findings

Moon-forming impact occurred at least 40 Myr after Solar System formation.

Estimated impact age is approximately 95 +/- 32 Myr after condensation.

Late accreted mass was less than 1% of Earth's mass, supporting a later impact date.

Abstract

According to the generally accepted scenario, the last giant impact on the Earth formed the Moon and initiated the final phase of core formation by melting the Earth'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 50 Myr (and possibly as late as around 100 My) after condensation. Here we show that a Moon-forming event at 40 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 it holds across a wide range of disk conditions. The concentration of highly siderophile elements (HSEs) in Earth'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 95 +/- 32 Myr since condensation. The possibility exists that some late projectiles were differentiated and left an incomplete HSE record in Earth's mantle. Even in this case, various isotopic constraints strongly suggest that the late-accreted mass did not exceed 1 per cent of Earth's mass, and so the HSE clock still robustly limits the timing of the Moon-forming event to significantly later than 40 My after condensation.