The prevalence of precious metals in the mantle and crust of the Earth today may be the result of cosmic collisions and storms of "iron rain" as our planet was forming, scientists say.
Powerful collisions between the young Earth and iron-rich objects within the solar system would have resulted in significant amounts of iron vapor, a study by researchers at the Lawrence Livermore National Laboratory in Berkeley, Calif., suggests.
This vapor would have settled onto the earth as an "iron rain" that may also have brought many iron-loving elements like gold, platinum and palladium into Earth's rocky outer shell, explaining why they became common enough for humans to mine and use, they report in the journal Nature Geoscience.
However, scientists have never had good model for what happens to colliding bodies when impact speeds are high enough to put materials under the resulting extreme pressures, they acknowledge.
"One major problem is how we model iron during impact events, as it is a major component of planets and its behavior is critical to how we understand planet formation," says LLNL scientist Richard Kraus said. "In particular, it is the fraction of that iron that is vaporized on impact that is not well understood."
To find out, Kraus and his colleagues used a machine at the Sandia National Laboratory in New Mexico that utilizes magnetic fields to accelerate metals to extreme speeds.
They fired small iron samples at aluminum plates at impact speeds of 30,000 miles per hour, with the collisions sending powerful shock waves into the iron, making it compress, heat up and turn to vapor.
They discovered it took much less pressure to vaporize iron than had previously been estimated, causing them to rethink their picture of the early Earth, where incoming meteors would have been pulverized on impact, throwing up huge plumes of vaporized iron and rock dust.
"This causes a shift in how we think about processes like the formation of Earth's iron core," Kraus says. "Rather than the iron in the colliding objects sinking down directly to the Earth's growing core, the iron is vaporized and spread over the surface within a vapor plume. After cooling, the vapor would have condensed into an iron rain that mixed into the Earth's still-molten mantle."
Experts said the finding goes a long way to explaining how the Earth's mantle and crust are surprising rich in precious metals, brought there by the iron "rain."
"The reason we're able to mine gold and make jewelry out of it, and mine palladium and make catalytic converters, is because the [crust and mantle] silicates have much higher abundances of these elements than one might expect," says Richard Walker at the University of Maryland. "This [study] is a pretty good way of explaining how they got here and why they're not located 2900 kilometers below your feet in the core."
