Synopsis

# Liquid Iron’s Density in Extreme Conditions

Physics 13, s51
The density of liquid iron is measured experimentally at conditions that match those inside Earth.

Finding the composition of Earth’s core is vital to understanding how our planet formed and how it has evolved over time. Scientists have inferred the core’s composition—largely thought to be made from iron—from seismic data and from theoretical models. But the properties of liquid iron under the core’s extreme temperature and pressure conditions were experimentally unconfirmed. Now, Yasuhiro Kuwayama of the University of Tokyo and colleagues report measurements of liquid iron’s density in experiments that replicate the outer core’s conditions. They find that Earth’s outer core is about 7.5% less dense than pure liquid iron.

The density of liquid iron has been measured in so-called dynamic compression experiments, which induce high-pressure conditions in the material for a few microseconds at most. But researchers are concerned that such short-duration experiments may poorly reproduce the conditions inside Earth. The short duration also makes it challenging to probe the material while it is at high pressure. Static compression experiments, which Kuwayama and colleagues performed, can subject materials to high pressure for years, potentially solving these problems.

In the experiments, the team squeezed a $10\text{-}𝜇\text{m}$-diameter drop of liquid iron between two diamond anvils and then heated the liquid with an infrared laser. The setup allowed them to achieve pressures of up to 116 GPa and temperatures of 4350 K. To determine the density of the iron, the team used x-ray scattering measurements.

Comparing the measured densities to estimates of Earth’s density from seismic data, the team calculates that the outer core is about 7.5% less dense than pure liquid iron. This difference is slightly larger than past estimates and suggests that the core contains a higher fraction of lighter elements than previously thought.

This research is published in Physical Review Letters.

–Erika K. Carlson

Erika K. Carlson is a Corresponding Editor for Physics based in Brooklyn, New York.

## Subject Areas

Condensed Matter Physics

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