Discover Earth

The solid iron core of the Earth is not as it appears. Recent scientific discoveries have revealed that the innermost sphere of our planet is not smooth, but rather textured, and undergoes a change in direction every seven decades after ceasing its rotation.

In a new and unexpected study, a team of researchers has proposed an explanation for why Earth's solid iron core is slightly more pliable than anticipated: the movement of its atoms. Located approximately 5,100 kilometers beneath the Earth's surface, the tightly packed iron atoms in the inner core form a hexagonal structure that is subjected to immense pressure and high temperatures.

Seismic observations have recently unveiled intriguing properties of Earth's inner sphere, resembling soft metals like lead and molten iron rather than the solid mass we had previously envisioned. Through a series of computer simulations and laboratory experiments, Youjun Zhang, a physicist at Sichuan University in China, along with colleagues from the United States and China, have suggested that this is due to the iron atoms within the inner core shifting positions within their hexagonal lattice structure.

Similar to individuals changing seats at a dinner table, the iron atoms migrate to neighboring positions without disrupting the underlying metallic structure of the iron. This phenomenon renders the core more malleable, as proposed by Zhang and his colleagues.

Zhang explains, "The solid iron deep within the Earth unexpectedly exhibits a softness because its atoms possess a greater degree of mobility than previously believed. This increased movement results in a less rigid inner core, making it more susceptible to shear forces."

Prior to this, scientists had utilized computer models to simulate the inner core of Earth, which typically consisted of fewer than one hundred atoms arranged in a repeating hexagonal structure.

Additionally, some researchers proposed that the presence of molten pockets within Earth's inner core could potentially account for certain observed characteristics.

However, Zhang and colleagues suggest that these pockets were likely squeezed out as the core solidified, and thus far, no comprehensive theory has been able to fully explain the peculiar flexibility of Earth's inner sphere.

In order to broaden their understanding of lattice dynamics, Zhang and colleagues employed a supercomputer and a machine learning algorithm to simulate a significantly larger atomic environment, consisting of over 10,000 atoms.

The researchers inputted data from laboratory experiments conducted under high pressure and temperature conditions, which aimed to replicate the conditions found in Earth's inner core.

Simulations of the closely-packed lattice structure, conducted at pressures ranging from 230 to 330 GPa and temperatures slightly below the melting point of iron, indicate that the iron atoms exhibit a pattern of collective motion. This motion involves one atom moving out of its equilibrium position and subsequently pushing its neighboring atoms along.

This rapid diffusion occurs within picoseconds, which is equivalent to one trillionth of a second, and does not disrupt the lattice structure. Instead, the atoms vibrate in such a way that the iron core behaves like an extremely malleable solid.

It is important to note that these findings are based on theoretical calculations of a substance that scientists are unable to directly sample, and can only infer its properties from a distance. Despite these limitations, the results align well with seismic observations.

Senior author Jung-Fu Lin, a geoscientist at the University of Texas, states, "Now, we have gained insight into the fundamental mechanism that will aid us in understanding the dynamic processes and evolution of Earth's inner core."

The study has been published in the Proceedings of the National Academy of Sciences (PNAS).