Why Earth Didn't Absorb the Gifts from Space express
In its youth, Earth resembled a beehive: it grew by sticking together from countless rocky chunks. When the main construction was over, a late swarm of meteorites rained down on the young planet — the so-called late veneer. These impacts, scientists thought, brought platinum, gold, and other rare metals to Earth's upper layers. Their excess in the mantle long served as evidence: here are the traces of cosmic visitors.
But new work turns this picture upside down. The authors traced the fate of metallic droplets from a meteorite impact. A drop of honey dropped into a glass of water quickly sinks to the bottom. Likewise, liquid iron from a large body: it pierces the mantle and goes into Earth's core. For bodies larger than a kilometer across, the metal almost completely disappears from the mantle. So large meteorites could not have left a noticeable trace.
To deliver the required amount of rare metals via small bodies, one would have to assume an incredibly large number of large ones, but their metal would have been lost to the core. A contradiction arises: the observed richness of the mantle does not match the simple picture of the late bombardment. The way out of the impasse is either that large bodies fragmented into fragments smaller than a hundredth of a millimeter, or that Earth received more oxidized material, similar to carbonaceous meteorites.
This paradox makes one wonder: maybe the late veneer was not what we imagined. The composition and sizes of the impacting bodies could have been different, and so the story of Earth's growth still awaits its detectives.
Original abstract
as written by the authors arXiv:2603.17961
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.
If you collected all the platinum, gold, and other rare metals from Earth's mantle, you'd get a layer only about 5 meters thick, but even its origin is now in question.