Scientists detect signs of a hidden structure within the Earth’s core

While most of us take the ground beneath our feet for granted, written within its complex layers, like the pages of a book, is the history of Earth. Our history.

Now researchers have found more evidence for an entirely new chapter deep within Earth’s past: Earth’s inner core appears to have another, even more inner core within it.

“Traditionally we have been taught that the Earth has four main layers: the crust, the mantle, the outer core and the inner core,” explained Australian National University geophysicist Joanne Stephenson.

Our knowledge of what lies beneath the earth’s crust has been inferred primarily from what volcanoes have disclosed and whispered seismic waves. From these indirect observations, scientists have calculated that the fiery hot inner core, with temperatures exceeding 5,000 degrees Celsius (9,000 Fahrenheit), constitutes only one percent of the Earth’s total volume.

Now, Stephenson and his colleagues have found more evidence that the Earth’s inner core may have two distinct layers.

“It’s very exciting, and it could mean we have to rewrite the textbooks!” she added.

The team used a search algorithm to track and match thousands of models of the inner core with data observed over many decades on how long it takes for seismic waves to travel through the Earth, collected by the International Seismological Center.

Differences in the trajectories of seismic waves through the layers of the Earth. (Stephenson et al., Journal of Geophysical Research: Solid Earth, 2021)

So what’s down there? The team analyzed some models of the anisotropy of the inner core, how differences in the composition of its material alter the properties of seismic waves, and found that some were more likely than others.

While some models think that the inner core material channels seismic waves more quickly parallel to the equator, others argue that the mix of materials allows faster waves more parallel to the Earth’s axis of rotation. Even then, there are discussions about the exact degree of difference at certain angles.

This study did not show much variation with depth in the inner core, but found that there was a change in the slow direction at an angle of 54 degrees, with the faster direction of the waves parallel to the axis.

“We found evidence that may indicate a change in the structure of iron, suggesting perhaps two separate cooling events in Earth’s history,” Stephenson said.

“The details of this great event are still a bit mysterious, but we have added another piece of the puzzle when it comes to our knowledge of the inner core of the Earth.”

These new findings may explain why some experimental evidence has been inconsistent with our current models of Earth’s structure.

The presence of an innermost layer has been suspected for some time, with indications that the iron crystals that make up the inner core have different structural alignments.

“We are limited by the distribution of global earthquakes and receptors, especially in the antipodes,” the team wrote in their paper, explaining that the missing data diminishes the certainty of their conclusions. But their conclusions align with other recent studies on the anisotropy of the innermost inner core.

A new method currently in development could soon fill in some of these data gaps and allow scientists to corroborate or contradict their findings and hopefully translate more stories written within this first layer of Earth’s history.

This research was published in the Geophysical Research Journal.


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