Discovery of rapid “mass movement” of iron atoms in the Earth’s solid inner core

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

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

But even here, the researchers found there is room to maneuver.

A study by the University of Texas at Austin and collaborators in China found that certain groups of iron atoms in the Earth’s inner core are able to move quickly, changing places in a fraction of a second while maintaining the basic metallic structure of iron. A type of movement known as “mass movement” that resembles dinner guests changing their seats at the table.

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

Effects of the Earth’s magnetic field

The results, obtained through laboratory experiments and theoretical models, indicate that atoms in the inner core move much more than previously thought.

The results could help explain several interesting properties of the inner core that have long puzzled 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 the basic mechanism that will help us understand the dynamical processes and evolution of the Earth’s inner core,” said Jong-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 in the Earth’s inner core are expected to move in 10 picoseconds. Lines represent a path corn It also moves over time. The model is based on an artificial intelligence algorithm that represents tens of thousands of atoms. One picosecond is one trillionth of a second. Credit: Zhang et al.

Methods and results

It is impossible for scientists to take direct samples from the Earth’s inner core due to extremely high temperatures and pressures. So, Lin and his collaborators recreated it in miniature in the laboratory by taking a small iron plate and firing 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 the atoms in the inner core.

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

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

Co-lead author Jong Fu “Avo” Lin holds a model of iron atoms arranged in a hexagonal structure believed to occur in the Earth’s inner core. Credit: Jong-Fu Lin/UT Jackson College of Geosciences

Atomic motion explains seismic measurements

The researchers said atomic motion could explain why seismic measurements of the inner core show a softer, more flexible environment than would be expected at such pressures, said co-author Yujun Zhang, a professor at Sichuan University.

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

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

Role in Earth’s geodynamo energy

About half of the geodynamo’s energy that generates Earth’s magnetic field can be attributed to the inner core, according to the researchers, while the outer core makes up the rest. New insight into inner core activity at the atomic level could help inform future research into how energy and heat are generated in the inner core, how they relate to the dynamics of the outer core, and how they work together to generate the planet’s magnetic field. This is the key ingredient for a habitable planet.

Reference: “Collective motion in hcp-Fe in Earth’s interior 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 Gong Fu Lin, October 2, 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.