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 favor of this explanation.
A team led by Matthias Willbold of the University of Bristol, U.K., 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.
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.
"It is so far the best isotopic or geochemical evidence that late bombardment ever happened," he added.
"Our ability to measure these (isotopes) precisely enough to see these differences is just opening a totally new window into early planet formation," said Richard Carlson of the Carnegie Institution of Washington who has also studied early Earth using isotopes.
Read more at Discovery News
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