Characterizing the iron and nickel in the Earth’s core

The Earth’s inner core is thought to consist of an iron-nickel alloy with small amounts of lighter elements. Observations of seismic waves probe the relationship between pressure and density in the core, and interpreting those results depends on understanding the same relationship for iron and nickel individually.

Experiments assessing the metals have been limited to 300 GPa, despite pressures in the inner core ranging from 330 to 365 GPa. Using the latest pressure scale, and a diamond anvil cell (DAC) to achieve high pressures, Hirao et al. obtained the equations of state for iron and nickel at conditions mimicking the center of Earth. They found the densities of iron and nickel in the inner core had been overestimated by 2.3% and 1.7%, respectively.

“If accurate iron and nickel equations of state can be determined, influential knowledge on the kinds and contents of the light elements in the core can be obtained,” said author Yuichi Akahama.

To generate pressures reminiscent of the Earth’s core, the team used a DAC to compress a sample and compared it to platinum, a reference material with a known pressure scale. They optimized the design by introducing a double-beveled geometry, obtaining low levels of diamond impurities, and preventing microcracks on the culet surfaces.

The researchers developed synchrotron radiation X-ray focusing techniques for the generation of highly stable micro-focused X-ray beams, which allowed them to collect high-quality data from a tiny sample.

“The next main target is to extend the accessible pressure range over 400 GPa,” said Akahama. “Recently, many super-Earths, or rocky planets more massive than Earth, have been discovered with a pressure range beyond that of Earth.”

Source: “Equations of state of iron and nickel to the pressure at the center of the Earth,” by Naohisa Hirao, Yuichi Akahama, and Yasuo Ohishi, Matter and Radiation at Extremes (2022). The article can be accessed at https://doi.org/10.1063/5.0074340 .