Rapid “Collective Motion” of Iron Atoms Discovered in Earth’s Solid Inner Core

Charged ions interacting with the Earth’s magnetic field often create auroras near the planet’s poles. The aurora australis or the “southern lights,” are captured here by the NASA IMAGE satellite. Credit: NASA

A new study discovered rapid “collective motion” of iron atoms in Earth’s inner core. This movement may explain the core’s unexpected softness in seismic data and has implications for understanding Earth’s magnetic field generation.

The iron atoms that make up the Earth’s solid inner core are tightly jammed together by astronomically high pressures — the highest on the planet.

But even here, there’s space for wiggle room, researchers have found.

A study led by The University of Texas at Austin and collaborators in China found that certain groupings of iron atoms in the Earth’s inner core are able to move about rapidly, changing their places in a split second while maintaining the underlying metallic structure of the iron — a type of movement known as “collective motion” that’s akin to dinner guests changing seats at a table.

A model of iron atoms on the move in Earth’s inner core. The model demonstrates how iron atoms are expected to move about in the Earth’s inner core over 10 picoseconds. One picosecond is one trillionth of a second. Credit: Zhang et al.

Implications for Earth’s Magnetic Field

The results, which were informed by laboratory experiments and theoretical models, indicate that atoms in the inner core move around much more than previously thought.

The findings could help explain numerous intriguing properties of the inner core that have long vexed scientists. They could also help shed light on the role the inner core plays in powering Earth’s geodynamo — the elusive process that generates the planet’s magnetic field.

“Now, we know about the fundamental mechanism that will help us with understanding the dynamic processes and evolution of the Earth’s inner core,” said Jung-Fu Lin, a professor at the UT Jackson School of Geosciences and one of the study’s lead authors.

The study was published on October 2 in the journal Proceedings of the National Academy of Sciences.

A clip from a scientific model showing how iron atoms are expected to move about in the Earth’s inner core over 10 picoseconds. The lines represent the path of the atom as it moves over time. The model is based on an AI algorithm accounting for tens of thousands of atoms. One picosecond is one trillionth of a second. Credit: Zhang et al.

Methods and Findings

It’s impossible for scientists to directly sample the Earth’s inner core because of its extremely high temperatures and pressures. So, Lin and collaborators re-created it in miniature in the lab by taking a small iron plate and shooting it with a fast-moving projectile. The temperature, pressure, and velocity data collected during the experiment was then put into a machine-learning computer model of atoms in the inner core.

Scientists think that iron atoms in the inner core are arranged in a repeating hexagonal configuration. According to Lin, most computer models portraying the lattice dynamics of iron in the inner core show only a small number of atoms — usually fewer than a hundred. But using an AI algorithm, the researchers were able to significantly beef up the atomic environment, creating a “supercell” of about 30,000 atoms to more reliably predict iron’s properties.

At this supercell scale, the scientists observed groups of atoms moving about, changing places while still maintaining the overall hexagonal structure.

Co-lead author Jung-Fu “Afu” Lin holding a model of iron atoms arranged in the hexagonal structure thought to occur in the Earth’s inner core. Credit: Jung-Fu Lin / UT Jackson School of Geosciences

Atomic Movement Explains Seismic Measurements

The researchers said that the atomic movement could explain why seismic measurements of the inner core show an environment that’s much softer and malleable than would be expected at such pressures, said co-lead author Youjun Zhang, a professor at Sichuan University.

“Seismologists have found that the center of the Earth, called the inner core, is surprisingly soft, kind of like how butter is soft in your kitchen,” he said. “The big discovery that we’ve found is that solid iron becomes surprisingly soft deep inside the Earth because its atoms can move much more than we ever imagined. This increased movement makes the inner core less rigid, weaker against shear forces.”

The researchers said that searching for an answer to explain the “surprisingly soft” physical properties reflected in the seismic data is what motivated their research.

Role in Earth’s Geodynamo Energy

About half of the geodynamo energy that generates the Earth’s magnetic field can be attributed to the inner core, according to the researchers, with the outer core making up the rest. The new insight on inner core activity at the atomic scale can help inform future research on how energy and heat are generated in the inner core, how it relates to the dynamics of the outer core, and how they work together to generate the planet’s magnetic field that is a key ingredient for a habitable planet.

Reference: “Collective motion in hcp-Fe at Earth’s inner core conditions” by Youjun Zhang, Yong Wang, Yuqian Huang, Junjie Wang, Zhixin Liang, Long Hao, Zhipeng Gao, Jun Li, Qiang Wu, Hong Zhang, Yun Liu, Jian Sun and Jung-Fu Lin, 2 October 2023, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2309952120

The study was funded by the National Natural Science Foundation of China and the Geophysics Program of the National Science Foundation.