While most of us take the ground beneath our feet for granted, written in its complex layers, like pages of a book, is the history of the earth. Our history.
Now, researchers have found more evidence for a whole new chapter deep in Earth’s past – Earth’s inner core appears to contain another even more inner core.
“Traditionally, we have been taught that the Earth has four main layers: the crust, mantle, outer core and inner core,” explained geophysicist Joanne Stephenson of Australia’s National University.
Our knowledge of what lies beneath Earth’s crust derives largely from what volcanoes have revealed and seismic waves whispered. From these indirect observations, scientists have calculated that the scorching hot inner core, with temperatures in excess of 5,000 degrees Celsius (9,000 Fahrenheit), makes up just one percent of Earth’s total volume.
Now Stephenson and colleagues have found more evidence that Earth’s inner core may have two different layers.
“It’s very exciting – and it could mean rewriting the textbooks!” she added.
The team used a search algorithm to search thousands of models of the inner core and match them with observed data over many decades on how long seismic waves take to travel through Earth, collected by the International Seismological Center.
Differences in seismic wave paths through strata. (Stephenson et al., Journal of Geophysical Research: Solid Earth, 2021)
So what’s down there? The team looked at some models of the inner core anisotropy – how differences in the composition of the material change the properties of seismic waves – and found that some were more likely than others.
While some models think that the material of the inner core channels seismic waves more quickly parallel to the equator, others argue that the mix of materials allows for faster waves that are more parallel to the Earth’s rotational axis. Even then, there are arguments about the exact degree of difference at certain angles.
This study did not show much variation with the depth in the inner core, but it did find that there was a change in the slow direction to an angle of 54 degrees, with the faster direction of waves running parallel to the axis.
“We found evidence that could indicate a change in the structure of iron, suggesting perhaps two separate cooling events in Earth’s history,” said Stephenson.
“The details of this great event are still a bit of a mystery, but we’ve added one more piece to the puzzle when it comes to our understanding of the Earth’s inner core.”
These new findings may explain why some experimental evidence is inconsistent with our current models of Earth’s structure.
The presence of an innermost layer has been suspected for some time, with indications that iron crystals that make up the innermost core have different structural alignments.
“We are limited by the spread of global earthquakes and receivers, especially at polar antipodes,” the team wrote in their paper. By explaining that the missing data diminishes the certainty of their conclusions. But their conclusions are in line with other recent studies on inner core anisotropy.
A new method currently under development could soon fill some of these data gaps and allow scientists to confirm or contradict their findings and hopefully translate more stories written in this early layer of Earth’s history.
This research is published in the Journal of Geophysical Research