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

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.

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

The paper "Highly Siderophile Elements in the Earth's Mantle as a Clock for the Moon-forming Impact" presents a detailed examination of the timing of the Moon-forming impact, a critical event in Earth's history. This study utilizes the abundance of highly siderophile elements (HSEs) in Earth's mantle as a novel chronometer to date the event, offering an alternative to traditional radiometric dating methods. The research team, consisting of Seth A. Jacobson et al., applies a combination of geochemical analysis and extensive N-body simulations to define the temporal constraints on the Moon-forming impact.

Key Findings

• HSE Constrained Moon-formation Age: The authors determine a Moon-formation age of approximately 95 ± 32 Myr after the condensation of the Solar System's first solids. This estimate is derived from HSE concentration measurements in Earth's mantle, which are linked to the mass added during Late Accretion. Their findings challenge earlier proposals suggesting a Moon-forming event between 30 Myr and 50 Myr after condensation, ruling out these timelines at a 99.9% confidence level.
• N-body Simulation Insights: The study employs a suite of 259 total simulations combining classical scenarios and the Grand Tack model. These simulations reveal a statistically significant correlation between the timing of Earth's last giant impact and the subsequent late-accreted mass. The Grand Tack model, characterized by an early migration pattern of Jupiter and Saturn, more accurately reproduces the current mass-orbit distribution of terrestrial planets compared to classical models.
• Late Accretion and HSE Correlation: The simulated late-accreted mass timelines are consistent with HSE-based estimates, particularly under the Grand Tack scenario. The late accreted mass is firmly constrained to less than 0.01 Earth masses, ensuring compatibility with geochemical observations.

Implications

This research provides compelling evidence that the Moon-forming impact occurred significantly later than 40 Myr post-condensation, challenging conventional estimates based on radiometric dating. The implications here are twofold:

1. Terrestrial Planet Formation: The timing influenced by HSEs suggests a predominantly embryo-populated disk during accretion, shifting away from planetesimal-dominated models. This has profound implications for the understanding of planetary accretion dynamics and the initial conditions of the protosolar disk.
2. Isotopic Homogeneity: The similarity in isotopic composition between Earth and the Moon indicates that large, homogenous planetesimals predominantly contributed to Late Accretion, rather than achondritic bodies. This notion aligns with enstatite chondrite-like composition limits drawn from isotopic studies.

Future Directions

The study advocates for refining isotopic analyses, specifically of tungsten, to narrow the constraints on Late Accretion mass and further affirm the Moon-forming impact timing. Potential considerations for Late Accretion models incorporating projectile differentiation and imperfect accretion could unlock new perspectives on early Earth's geological evolution.

Overall, Jacobson et al.'s work underscores the importance of geochemical markers such as HSEs in providing robust temporal estimates for pivotal events in Earth's history. This approach not only corroborates certain radiometric chronometer results but also sets a new precedent for planetary differentiation and accretion studies.