The mystery of the disappearing iridium

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? » 2/15/13 1:40pm 2/15/13 1:40pm