Geologists have long speculated that four and a half billion years ago, the Earth was a cold mass of rock mixed with iron metal which was melted by the heat generated from the impact of massive planet-sized objects, allowing the iron to separate from the rock and form the Earth's core. Researchers recreated the extreme pressure and temperature of this process, subjecting a similar mixture to temperatures above 2,000 °C, and measured the composition of the resulting rock and iron.
Because the rock became void of the metal in the process, the scientists speculate that the same would have occurred when the Earth was formed, and that some sort of external source – such as a rain of extraterrestrial material – contributed to the presence of some precious metals in Earth's outer rocky portion today.
"The notion of extraterrestrial rain my also explain another mystery, which is how the rock portion of the Earth came to have hydrogen, carbon and phosphorous – the essential components for life, which were likely lost during Earth's violent beginning." link
Ref: Core formation and metal–silicate fractionation of osmium and iridium from gold. 2009. J.M. Brenan and W.F. McDonough. Nature Geoscience, published online October 18.
