Meteorites Pummeled Earth, Delivering Gold

Your wedding ring, the gold chain around your neck… even the platinum in your catalytic converter. For all of these you can thank a slew of meteorites that pelted Earth around 3.9 billion years ago, says new research. 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 Earth stripped of its precious metals.

Yet as hopeful ’49-ers knew, Earth’s crust is laced with these enticing elements. Geologists have posed several theories to explain this puzzle, but one suggests that Earth was bombarded with meteorites between 3.8 and 4 billion years ago, studding the early crust with our favorite shiny metals. These metals then became incorporated into the modern mantle over time. This idea is supported by the presence of craters on the moon, which date back to the same time, suggesting that the moon was also hit. Now, research published today in Nature, provides further evidence in favour of this explanation.

A team led by Matthias Willbold of the University of Bristol, U.K., sampled ancient rocks from south-west 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. The researchers found distinct differences in the concentrations of particular tungsten isotopes in each type.

“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.

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