Earth’s mantle enriched by iron rain

Vaporization of iron in violent collisions between the early Earth and other objects in the Solar System may explain the iron-rich composition of the Earth's mantle, reports a study published online in Nature Geoscience. It was previously assumed that iron from these impacts would melt and quickly sink into the Earth's core.

High-speed impacts between two solid objects produce high-pressure shockwaves that can compress solid material. After the shockwave has passed, if the pressure was sufficiently high, the material will vaporize.

Richard Kraus and colleagues subjected iron samples to extremely high shock pressures by using the Sandia National Laboratories Z-machine, the world's most powerful radiation source, to slam aluminium plates into iron samples at extremely high velocities. They found that the shock pressure required to vaporize iron is much lower than previously thought, and could be readily achieved in the high-speed impacts between the early Earth and other objects in the Solar System towards the end of planet formation. The authors propose that the iron cores of these objects were vaporized by shockwaves generated on impact and the resulting plume of vapour was distributed around the Earth. After cooling, the vapour would then have condensed into an iron rain that then mixed into the Earth’s still-molten mantle.

This process may also explain why the Moon, which is thought to have formed by this time, lacks iron-rich material despite being exposed to similarly violent collisions. The authors suggest that the Moon's reduced gravity could have prevented it from retaining most of the vaporized iron.

In an accompanying News and Views article, William Anderson writes that "the study emphasizes the role that high-velocity impact processes played in the evolution of the Earth and planets."

doi: 10.1038/ngeo2369

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