The efficient delivery of highly-siderophile elements to the core creates a mass accretion catastrophe for the Earth

TL;DR

This paper challenges traditional models of Earth's late veneer by analyzing how highly siderophile elements (HSEs) are delivered and retained, suggesting that large impactors or specific fragmentation processes are necessary to explain observed HSE levels.

Contribution

It provides an analytical model showing that metal from impactors larger than 1 km sinks to Earth's core, contradicting the observed mantle HSEs, and proposes mechanisms to resolve this paradox.

Findings

Large impactors' metal sinks to Earth's core, leaving no HSE in mantle.

A collisional size distribution implies implausibly large mass from big impactors.

Disruption of impactor cores or oxidized material delivery may explain HSE signatures.

Abstract

The excess abundance of highly siderophile elements (HSEs), as inferred for the terrestrial planets and the Moon, is thought to record a `late veneer' of impacts after the giant impact phase of planet formation. Estimates for total mass accretion during this period typically assume all HSEs delivered remain entrained in the mantle. Here, we present an analytical discussion of the fate of liquid metal diapirs in both a magma pond and a solid mantle, and show that metals from impactors larger than approximately 1 km will sink to Earth's core, leaving no HSE signature in the mantle. However, by considering a collisional size distribution, we show that to deliver sufficient mass in small impactors to account for Earth's HSEs, there will be an implausibly large mass delivered by larger bodies, the metallic fraction of which lost to Earth's core. There is therefore a contradiction between observed concentrations of HSEs, the geodynamics of metal entrainment, and estimates of total mass accretion during the late veneer. To resolve this paradox, and avoid such a mass accretion catastrophe, our results suggest that large impactors must contribute to observed HSE signatures. For these HSEs to be entrained in the mantle, either some mechanism(s) must efficiently disrupt impactor core material into $\leq0.01$ mm fragments, or alternatively Earth accreted a significant mass fraction of oxidised (carbonaceous chondrite-like) material during the late veneer. Estimates of total mass accretion accordingly remain unconstrained, given uncertainty in both the efficiency of impactor core fragmentation, and the chemical composition of the late veneer.