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.