Early in the formation of the Earth, some forms of the element chromium separated and disappeared deep into the planet's core, according to a new study in Science.

The team studied a class of meteorites called chondrites, which are leftovers from the formation of the solar system more than 4.5 billion years ago.    By making very accurate measurements of chromium isotopes in the meteorites compared to Earth rocks and comparing them to theoretical predictions, the researchers were able to show for the first time that lighter isotopes of chromium preferentially go into the core.   From this the team inferred that some 65 percent of the missing chromium is most likely in the Earth's core.

Their measurements of chromium isotopes in meteorites and comparison to rocks from the Earth's crust allowed them to generate a simulation of Earth's early environment.

As well as adding shiny, rust-proof surfaces to metalwork, chromium adds color to emeralds and rubies. It exists as four stable (nonradioactive) isotopes with atomic masses of 50, 52, 53 and 54.   It has been known for decades that chromium is relatively underrepresented in the Earth's mantle and crust, said
Qing-Zhu Yin, professor of geology at UC Davis and co-author of
the paper.  Speculation was that it evaporated into space or got sucked into the Earth's deep core at some point.

The separation must have happened early in the planet-building process, they now believe, probably in the multiple smaller bodies that assembled into the Earth or when the Earth was still molten but smaller than today.   

The finding will help scientists understand the early stages of planet formation, they conclude.  The work was funded by grants from NASA and the National Science Foundation.