Earth’s core is oddly squishy and we may now know why

Earth’s iron-rich inner core may owe some of its surprising softness to the motion of atoms, suggest experiments with iron at high temperature and pressure coupled to AI simulations

Earth’s core is oddly squishy and we may now know why
The iron-rich core at the center of the Earth is squishy like rubber (Credit: Maksym Yemelyanov/Alamy)

Earth’s inner core is surprisingly soft, and wiggling iron atoms may help explain why.

The innermost part of our planet is a solid iron ball smaller than the moon and almost as hot as the surface of the sun. Researchers cannot sample this inner core directly, but studies of seismic waves that pass through it indicate that it is unexpectedly soft – probably more like rubber than cast iron.

Jung-Fu Lin at the University of Texas at Austin and his colleagues wanted to understand why. To do so, they fired a small projectile at 2-centimetre-diameter discs of iron, each disc being under 1.5 mm thick. Upon impact, the iron atoms briefly – for a few hundred nanoseconds – experienced pressures up to 2.28 million atmospheres and reached temperatures of 4947 °C. These conditions are not as extreme as those found in the core, where the pressure is 3.6 million atmospheres and the temperature is 5200° C, but they are close enough to give an indication of the way the iron core behaves

Lin and his colleagues used a laser to measure how fast sound waves move through the impacted metal to determine its material properties, such as strength and softness. Their measurements matched the findings from seismological studies of the inner core, indicating that its softness can probably be explained by assuming it is essentially a lump of pure iron.

The researchers also used their measurements to run a computer simulation of the iron’s behaviour to uncover why it has these properties under extreme conditions.

In the past, such simulations were limited to modelling the behaviour of tens or hundreds of iron atoms, but the team used machine learning to increase that number to about 30,000. This more realistic simulation revealed that iron atoms which begin as a hexagonal crystal manage to wiggle from one spot in the structure to another, sort of like a game of musical chairs, says Lin. The researchers believe this is exactly the kind of collective motion that causes the inner core’s softness.

Hrvoje Tkalčić at the Australian National University says the new work is a step forward in understanding the inner core, but some details concerning how sound waves propagate differently along various directions through the iron remain unclear. He says researchers could explain the inner core’s softness by “playing other games” in simulations, such as adding new elements or adjusting the temperature of the iron.

Lin says similar studies may even advance our understanding of cores of exoplanets. “The implications always stretch beyond our terrestrial boundaries to other planets: how their cores evolve, and how they generate and maintain their magnetic fields,” says Tkalčić.

Journal reference

PNAS DOI: 10.1073/pnas.2309952120

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