Early Earth was a violent place. City-size
planetesimals — rocky microworlds that clumped
together in the solar nebula — smashed into our
planet’s surface at incredible velocities and seeped down
to Earth’s iron core, depositing yet more iron. But now
it appears that iron is instead spattered throughout
our world’s mantle.
To figure out how this happened, Harvard University
planetary scientists paired with researchers working
at Sandia National Laboratories’ Z machine and
shot metal projectiles into tiny iron squares at up to
Through these impacts, researchers discovered that
iron vaporizes much easier than scientists had assumed.
Instead of sinking into the core, iron-rich planetesimals
vaporized on impact and spread out across the planet.
This iron rain explains the element’s distribution.
“We did not expect to find that at all. We were very
surprised based on the fact that previous models people
used for planet formation suggested a much higher
critical shock pressure,” says Richard Kraus, now a
scientist at Lawrence Livermore National Laboratory
and lead author on the paper published in March in
[This article originally appeared in print as "Blame It on the Iron Rain."]