Not all that glitters is gold. But Earth would have a lot less of the glittery stuff if not for a massive rain of meteors about 3.9 billion years ago, according to a new study.
Based on analysis of some of the world's oldest rocks, scientists have the first direct evidence that a cataclysmic meteor shower changed early Earth's chemical composition.
The find offers support for the theory that meteors delivered gold and other precious metals to infant Earth.
Gold Sank in "Magma Ball" Earth
The presence of precious metals in Earth's mantle and crust poses a puzzle, because these elements are attracted to iron. (Get an overview of Earth's layers).
When Earth first formed roughly 4.5 billion years ago, the planet was basically a ball of magma. As the planet cooled, denser material sank toward the center, eventually producing a core made mostly of iron.
But that means any iron-loving—or siderophile—elements present in the primordial magma should have also retreated toward the core.
In fact, based on the composition of meteorites thought to be akin to early Earth, our planet should have enough gold in its present-day core to cover the entire globe with a 12-foot-thick (4-meter-thick) layer of the precious metal.
"All that stuff disappeared into the core, but we still find some gold around [the surface]," said study co-author Matthias Willbold of the University of Bristol.
One possible answer for where the precious metals came from is that a "firestorm" of meteors called the terminal bombardment added a veneer of material to Earth's surface some 650 million years after the planet's formation.
(Related: "Earth Asteroid Bombardment Mystery Solved?")
Ancient Rocks Offer Chemical Clues
To find proof for this theory, Willbold and colleagues studied rock samples from the Isua Greenstone Belt in Greenland. (Related: "Oldest Rocks on Earth Discovered?" .)
Although the Greenstone rocks date to about 3.8 billion years ago—close to the time of the terminal bombardment—"the mantle source from which these rocks are coming is from 4.5 billion years ago," Willbold said.
That means the rocks should retain chemical signatures that predate the massive meteor shower.
By comparing those ancient rocks with modern ones, the researchers found that the two samples have different tungsten isotope ratios.
Tungsten-182 was produced only in the first 50 million years of the solar system. But the Greenland rocks have more tungsten-182 than tungsten-184, the version of the element more commonly found in modern rocks.
"These rocks that we found on Greenland are the only ones that show an anomalous tungsten condition," Willbold said—a sign that meteor impacts did in fact change Earth's surface composition.
In general, based on the rate of impacts during the terminal bombardment, meteors slamming into Earth may have added about half a percent of the material now in the planet's mantle, Willbold said.
That may not seem like much, but it works out to about 20 quintillion tons, he added.
The research on Earth's gold supply appears in this week's issue of the journal Nature.