A meteorite shower is the source of complex and heavy elements such as gold on the Earth, a new study indicates.
A team led by Matthias Willbold of the University of Bristol, UK, sampled ancient rocks from southwest Greenland that formed some of the Earth's earliest crust, predating the proposed bombardment, and compared those with newer rocks from other places representing the modern mantle.
They measured the isotopic composition of tungsten, a rare element which, like gold and other heavy precious metals, gravitated to Earth's centre during the formation of the core.
The research published in Nature says the different makeup of rocks from the Earth's mantle of 4.8 billion years ago and 3.8 billion years ago suggests a protracted meteor bombardment sometime around 3.9 billion years ago which laced the Earth's crust with billions of tons of dead-star residues that we now mine.
When Earth was forming, molten iron sank to its center, creating the planet's core. That iron carried with it a slew of iron-loving metals, including gold and platinum. In fact, the Earth's core is packed with enough precious metals to cover the entire planet four meters deep.
When atoms have the same chemical makeup but a varying number of neutrons, which changes an atom's mass, they are said to have different isotopes.
Minute variations can indicate both the origin and age of a mineral.
Isotope concentrations in rock samples of different ages indicate that the composition of the Earth's mantle changed after the planet was blasted with meteors 3.9 billion years ago.
Certain metals like gold, platinum, nickel, tungsten and iridium are attracted to iron, which comprises the Earth's core. So when the Earth first formed as a molten mass, all of these elements should have migrated to the core, leaving the outer layers of the Earth stripped of its precious metals.
This is a sort of a time capsule that gave us the possibility to calculate how much material had to be added to the Earth to satisfy the tungsten isotopic composition that we find in the Earth today, Willbold said.
The researchers calculated that about half a percent of the present-day mantle would have been added by meteorites.