When Earth was formed from large colliding rocks, the impacts released tremendous energy, melting the elements in the fledgling planet. The iron sank into the core, where it stayed, and the rock, mostly silicate, floated on the surface. So, too, siderophilesor those elements that are more soluble in ironare thought to have followed the iron inward. Vanadium and chromium apparently did, based on comparisons with chondrites, old meteorites left over from the rocks that created the planets. But other iron-loving elements, among them calcium and scandium, proved more abundant in the mantle surrounding Earth's core. Scientists have always had a hard time accounting for how these strange elements, known as refractory siderophiles, ended up where they did. But now they've got a theory. Jon Wade and Bernard J. Wood of the University of Bristol report on niobium in today's issue of Nature.

Based on its chemical properties, niobium should be a refractory siderophile, and so there should be as much of it in the mantle as is found in chondrites. Instead it is missing. Some geochemists have suggested that the rare element is hidden in a reservoir deep in the mantlealthough most are hard-pressed to explain how such a reservoir might have come to exist. But Wade and Wood went back to basics, studying niobium's behavior in the laboratory. They found that when subjected to the high pressures and temperatures like those in the center of the Earth, niobium acted more like normal siderophiles than the refractory kind. Thus, they conclude that it, too, dissolved in the iron core. Case solved.