..Scientists have long known that there's a mysterious amount of siderophile ("iron-loving") metals in Earth's mantle. Such metals, including gold, tend to affiliate with iron in their liquid forms. The best explanation has been that some sort of space object brought the elements to the planet just after it formed its core, but the exact nature of the impactor has been a matter of debate. Based on computer simulations, the new study says that a small number of enormous, random impacts roughly 4.5 billion years ago are the sources of Earth's iron-loving materials. These impactors were rocky objects left over...