Shiny Metal

Rare, precious metals may owe their presence in Earth’s upper crust to a bombardment of the infant planet by asteroids billions of years ago, according to a study.

Gerhard Schmidt from the University of Mainz, western Germany, carried out a 12-year investigation into impact sites left by meteorites, analysing the soil for traces of these precious metals, which are called highly siderophile elements (HSE). Metals in the HSE group include gold, platinum, palladium, iridium and ruthenium.

Schmidt compared these with samples from Earth’s mantle and crust; from Martian meteorites that have been found on Earth; and from analysis of HSE-rich rocks, brought back by the Apollo missions, found at impact sites on the Moon.

The startling similarities point to a “cosmochemical source” for terrestrial HSE, he said.

Schmidt was scheduled to present his work at the European Planetary Science Congress He calculates that around 160 large, metal-rich asteroids in the order of 20 kilometers in diameter would have been enough to provide the concentrations of HSE we see today.