Earth's mantle contains trace amounts of highly siderophile elements — platinum, iridium, osmium, gold. These elements preferentially dissolve in iron, so they should have been stripped from the mantle and pulled into the core during Earth's differentiation. Their presence in the mantle is explained by a “late veneer”: a final pulse of asteroid impacts after core formation was complete, delivering fresh siderophile material that stayed in the mantle because there was no longer a mechanism to remove it.
Anslow et al. (arXiv:2603.17961) identify a catastrophe in this explanation. Metal from impactors larger than about 1 kilometer sinks to the core rather than mixing into the mantle. The metal droplets are too dense and too large to remain suspended. Only impacts from bodies smaller than ~1 km deliver their metal to the mantle.
But asteroid size distributions are steep power laws: for every small impactor, there must be proportionally fewer large ones. To deliver enough siderophile material via small impactors, the total population must include large impactors too — which contribute mass to Earth but not to the mantle's siderophile budget, because their metal sinks. The mass of material needed to explain the mantle's siderophile abundance, when integrated over a realistic size distribution, implies an implausibly large total mass delivery to Earth.
The solutions are narrow. Either impactor cores fragment into particles smaller than 0.01 mm (preventing sinking), or the late veneer was composed of oxidized carbonaceous material (where siderophile elements are differently distributed). Both are possible but constrained. The mantle's platinum tells us something happened; the physics of sinking metal tells us the obvious story doesn't add up.