While most of us consider the earth beneath our feet, its complex layers contain Earth’s history, like the pages of a book. Our story. Research shows that there are little-known parts of this story deep in Earth’s past. In fact, the Earth’s inner core appears to have another, even more inner core.

“Traditionally, we have been taught that the Earth has four main layers: crust, mantle, outer core and inner core,” Australian National University geophysicist Joan Stevenson explained in 2021.

Our knowledge of what lies beneath the Earth’s crust comes largely from what volcanoes reveal and what seismic waves whisper. Based on these indirect observations, scientists estimate that the hot inner core, with temperatures above 5,000 degrees Celsius (9,000 Fahrenheit), accounts for only 1 percent of Earth’s total volume. But a few years ago, Stevenson and his colleagues found evidence that the Earth’s inner core may actually have two distinct layers.

“This is very exciting and might mean we have to rewrite the textbooks!” Stevenson explained then.

The team used a search algorithm to sift through thousands of models of the inner core and match them with decades of observational data on how long seismic waves take to travel through the Earth, collected by the International Center for Seismology.

So what’s down there? The team examined various patterns of anisotropy (how differences in the composition of the material change the properties of seismic waves) in the inner core and found that some were more likely than others.

Some models suggest that the material of the inner core produces faster seismic waves parallel to the equator, while others show that the mixture of material provides faster waves more parallel to the Earth’s rotation axis. Even then, there is debate about the exact degree of difference at certain angles.

The study here showed no significant change with depth in the inner core, but found a change in the slow direction, parallel to the axis, in the faster wave direction up to an angle of 54 degrees.

“We found evidence that may indicate a change in iron structure, suggesting possibly two separate cooling events in Earth history,” Stevenson said.

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

These findings may explain why some experimental data are inconsistent with our current models of the Earth’s structure; The existence of the innermost layer was previously suspected, with hints that the iron crystals forming the inner core have a different structural arrangement.

“We are limited by the distribution of global earthquakes and receivers, particularly at the antipodes of the poles,” the team wrote in their paper, explaining that the lack of data reduces the precision of their conclusions.

However, their findings are consistent with other studies of the anisotropy of the innermost nucleus.

Future research could fill some of these data gaps and allow scientists to confirm or reject their findings, hopefully translating more of the stories written in this early layer of Earth history. This study was published on: Journal of Geophysical Research.