Iron Rain Helped Form the Earth's Core... and Moon?

During the violent beginnings of early Earth some 4.6 billion years ago, it seems that weathermen could have forecast iron rain - storms that supposedly led to the formation of the Earth's core and even the Moon, according to a new study.

Scientists previously thought that as our infant planet was being bombarded by many iron-rich celestial objects, this metal mostly melted into the Earth's core. However, lead author Richard Kraus at the Lawrence Livermore National Laboratory in California and his colleagues wanted to better understand how iron behaves under such extreme conditions, and it takes for it to vaporize.

"We care about when iron vaporizes because it is critical to learning how Earth's core grew," co-author Sarah Stewart, from the University of California, Davis, said in a statement.

To recreate the ancient conditions that led to Earth's formation, the team used the Z-machine at the Sandia National Laboratory in Albuquerque, New Mexico, which can accelerate metals to extreme speeds using high magnetic fields.

They shot iron samples with aluminum plates at extremely high speeds and pressures - the plates were accelerated to between 50,000 and 60,000 kilometers an hour (31,068-37,282 mph). The resulting collisions sent powerful shock waves through the iron, causing it to compress, heat up and eventually turn to vapor.

The researchers found that the shock pressure required to vaporize iron is much lower than expected - about 40 percent below original estimates. This means that more iron was vaporized during Earth's formation than previously thought.

"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," Kraus explained. "This means that the iron can mix much more easily with Earth's mantle."

And when the vapor cooled, it would have condensed into an iron rain that mixed into Earth's still-molten mantle.

These findings may not only change scientists' understanding of the processes involved in Earth's core formation, but also why the Moon has much less metal in its surface. The Moon is thought to have broken off from the Earth after a major impact, and so in theory it would have a similar metal composition. However, the researchers suggest the Moon's reduced gravity could have prevented it from retaining most of the vaporized iron.

The results were published in the journal Nature Geoscience.

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