One of the ways that we know when and where comets came crashing into Earth is by checking the ground for iridium. Although this platinum-colored metal is common in space, Earth's supply of iridium seems to have vanished. Why is it so common out there, but so very rare down here?
Iridium is a silvery white metal that was first established as an element in the early 1800s when it was separated out from platinum. It often combines with platinum, and the combination is one of the more useful ones in the world. Iridium, although brittle and unworkable on its own, is incredibly resistant to corrosion of any kind. Putting it together with platinum makes for reliable electronics in those things that absolutely have to work under hard conditions. It's used on spark plugs in helicopters. It was also used at the bureau of weights and measures to form the international standard kilogram and meter. All in all, it has both cultural and scientific value.
On Earth it's most likely to be found at the sites of comet or meteor impacts. When geologists come across a geological layer with high iridium content, they know they've hit a spot in the fossil record when a giant meteor struck, and its impact kicked up a cloud of dust with iridium in it. Why do these space rocks have such a high level of it, while the Earth is impoverished?
The main clue to iridium's vanishing act is the fact that it is densest element possible. While others elements are larger, in terms of protons and neutrons, you can't beat iridium for team spirit. The individual atoms cluster together like they're repelling an invasion. The next clue is that it seems to team up almost exclusively with other dense elements, like platinum and iron. Both are elements that we tend to have to dig for.
If iridium came on space rocks, it had to be here since the beginning, but unlike other material, it couldn't stay up top. While the Earth was still no more than a molten aggregate of space rocks itself, iridium inevitably and irretrievably shifted farther and farther down towards its center. It also combined with iron - which did little to lighten it. Like Atlantis, it sank beneath the sea (and the crust and the mantel), leaving the surface of the Earth mostly iridium free. So while we do actually have plenty of iridium on Earth, it's nowhere we could get to it. Better to try to snatch it out of the sky.