Caption Earth layers, computer artwork.

Earth’s 2,200-kilometre-thick outer core (pale hemisphere) is not composed solely of iron, according to new experiments Credit: Gary Hincks/SPL

Geophysics

Putting the squeeze on liquid iron helps to reveal Earth’s make-up

Diamonds help to hold iron at conditions simulating the planet’s interior.

Scientists have subjected iron to the extreme conditions found in the middle of Earth, yielding new clues to the composition of the planet’s outer core.

Analysis of seismic energy travelling through Earth’s interior, along with other observations, suggests that its core is dense, metallic and probably made of iron. But geologists have not pinned down the core’s exact composition.

Iron is liquid at the high temperatures and pressures in Earth’s centre. In previous experiments, researchers quickly shocked iron to simulate these extreme conditions. But Yasuhiro Kuwayama at the University of Tokyo and his colleagues slowly squeezed iron between two diamonds. This technique helped the team to measure the iron’s density more precisely than previous experiments had.

Pure liquid iron is 7.5% more dense than Earth’s outer core, the team found. That means the iron in the outer core must be mixed with some lighter element or elements.