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

Seth A. Jacobson*, Alessandro Morbidelli, Sean N. Raymond, David P. O'Brien, Kevin J. Walsh, David C. Rubie

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

124 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)84-87
Number of pages4
JournalNature
Volume508
Issue number1
DOIs
StatePublished - 2014

ASJC Scopus subject areas

  • General

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