Iridium is abundant in space rocks but scarce on Earth due to its high density and affinity for bonding with iron. During Earth's formation, much of the iridium likely sank into the planet's core while it was still molten, resulting in lower concentrations in the crust. The total amount of iridium on Earth is believed to be significantly higher than what is currently observed in crustal rocks.
PREREQUISITESGeologists, planetary scientists, and anyone interested in the distribution of elements in planetary bodies.
So, the "missing" Iridium took a one way trip into Earth's core.It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum-group metals, the high density and tendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.