Berlin, September 24 : A new study has suggested that some of the world's rarest and most precious metals, including platinum and iridium, could owe their presence in the Earth's crust to iron and stony-iron meteorites, which may have been fragments of a large number of asteroids from the inner solar system.
Dr Gerhard Schmidt from the University of Mainz, Germany, has calculated that about 160 metallic asteroids of about 20 kilometres in diameter would be sufficient to provide the concentrations of these metals, known as Highly Siderophile Elements (HSE), found in the Earth's crust.
According to Dr Schmidt, "A key issue for understanding the origin of planets is the knowledge of the abundances of HSE in the crust and mantle of the Earth, Mars and the Moon."
"We have found remarkably uniform abundance distributions of HSE in our samples of the Earth's upper crust. A comparison of these HSE values with meteorites strongly suggests that they have a cosmochemical source," he added.
During a 12-year study, Dr Schmidt and colleagues have analyzed the concentrations of HSE at meteorite impact sites around the world, as well as in the samples from the Earth's mantle and crust.
In addition, he has compared the data from the Earth with data from impact breccias from the Moon brought by the Apollo missions and Martian meteorites, believed to be samples from the mantle and crust on Mars.
Although HSE were present in the nebula from which the Earth formed, as the young planet evolved and heated up they were stripped, along with other heavy elements, from the silicate mantle into the iron and nickel-rich metallic core.
A widely accepted theory is that HSE were added by meteorite impacts as a veneer of material over the Earth's surface after the core had formed, about 20-30 million years after the planet's accretion.
This could have been by the collision with a Mars-sized impactor that led to the formation of the Moon.
Different classes of meteorites have characteristic elemental ratios of HSE that give indications where in the Solar System they formed.
However, the characteristic ratios of HSEs in the Earth's upper mantle match up with theoretical predictions for asteroids formed in the Mercury-Venus region.
Dr Schmidt's study shows that the abundance ratios of HSE in the Earth's crust are much higher than those found in stony meteorites, known as chondrites, which represent the pristine material from the early Solar System.
The ratios of HSE found in the crust bear a much closer resemblance to iron or stony-iron meteorites, which are fragments of larger asteroids that have had enough internal heat in the past to form a molten metal core.