Surprise! Earth Has a Secret Hidden Layer

A new study provides the best evidence yet for a hidden layer deep within Earth’s solid inner core.

Earth’s inner core, made of solid iron and nickel, is roughly two-thirds the size of the moon.

The researchers used machine learning to pinpoint the most accurate model of the strange physical processes that shape our planet's inner core.

Earth’s core is a strange, strange place. For decades, scientists have suspected that, like Matryoshka stacking dolls, there may be an additional hidden layer within our planet’s solid iron and nickel inner core.

➡ You love weird f#@!-ing science. So do we. Let’s nerd out over this stuff together. Now, a new study has provided the strongest evidence yet for this mysterious layer—what researchers have dubbed Earth’s inner-inner core. The study, which appears in the Journal of Geophysical Research: Solid Earth, could help to rewrite Earth science textbooks and reshape our understanding of what lies beneath our feet.

By studying the properties of the inner core, researchers can work to answer key questions about how Earth’s protective magnetic field formed and what it may have in store for the future.

“That’s what protects us from solar winds that would basically cause life not to exist as we know it today,” study author Joanna Stephenson, a post-doctoral researcher in geophysics at Australian National University in Canberra, tells Pop Mech. “That means the inner core is actually really, really important for life.”

But investigating the layers below Earth’s surface is a tricky task. There’s no way to conduct fieldwork in Earth’s core, so scientists rely on signals from seismic waves, laboratory experiments on super-heated, pressurized rocks, and computer models that tie all that data together.

Cut to the Core “To interrogate the inner core, one has to look at it through a not-particularly transparent window of [3,200 miles] of rock and molten metal,” Quentin Williams, a geoscientist at U.C. Santa Cruz who wasn’t affiliated with this study, tells Pop Mech.

Below Earth’s crust and mantle lies the core. The outer core, a roughly 1,500-mile-thick liquid layer of rock, can reach temperatures of up to 9,932 degrees Fahrenheit. The inner core, made of solid iron and nickel, is roughly two-thirds the size of the moon and has a radius of about 758 miles.

“There’s a high chance that there’s multiple changes in Earth’s core, and this is just one that we’re able to see with the data,” Stephenson says.

Scientists analyze seismic readings to make sense of what things look like below the surface. When earthquakes rupture, they send out seismic shock waves that travel through Earth’s layers at different velocities. They slow down or speed up depending on the composition of the material they move through, as well as the direction in which they travel (parallel vs. perpendicular to the equator). Anisotropy, the study of how these waves move through Earth, can shed light on how the buried material flows. For her new study, Stephenson and her team compiled a dataset of about 100,000 deep earthquakes—ranging between magnitude four and seven at depths greater than 62 miles below the surface—whose waves zipped through the inner core. Next, the scientists applied an algorithm that sorted through thousands of models of waves traveling through the inner core in an effort to find the most accurate model that could explain anomalies reported in previous research.

In the upper part of the inner core, seismic waves that travel in the north-south direction travel more quickly than waves that travel parallel to the equator—sometimes by as much as 4 miles per hour faster. At around 375 miles from the center of Earth, things go sideways.

Waves traveling in the slow direction that were parallel to the equator in the upper portion of the inner core suddenly aren’t. In fact, the angle of the waves shifts by about 54 degrees. “What I observed is a very small kind of change, but it’s got big implications for what’s happening in the inner core,” Stephenson says.

Further Reading: The Best Books About Volcanoes 🌋 So what is happening inside the inner core? It’s tough to say for sure. One possibility is some dramatic event in Earth’s history completely changed the development of the inner core. (Previous studies have suggested a polar reversal of Earth’s magnetic field may be to blame.)

Stephenson says the results from her research pair nicely with previous studies that have found evidence of an additional mystery layer. A 2015 study in Nature Geoscience revealed that iron crystals within the inner-inner core are arranged along an east-west axis, whereas iron crystals in the upper part of the inner core are arranged along a north-south axis.

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An Enduring Mystery Still, the core of our planet is shrouded in uncertainty. There are still a lot of basic facts about the inner core—like, say, exactly how old it is—that researchers haven’t yet been able to puzzle out, Williams explains.

Scientists in this field face a scarcity of seismic data from certain parts of the globe. There aren’t a lot of earthquakes that occur near the polar regions, for example, so our current view of Earth’s guts is incomplete. “There’s a high chance that there’s multiple changes in Earth’s core, and this is just one that we’re able to see with the data,” Stephenson says.

Another way to piece together what happens below the surface is to recreate the conditions found at depth in a laboratory above ground. Mineral physicists heat up and squeeze rocks to extreme temperatures and pressures in an effort to learn more about the physical properties of buried rock—data that can then be plugged into computer models.

Ultimately, studying Earth’s inner core and its inner-inner core helps scientists learn more about the formation of Earth’s magnetic field, which is generated in Earth’s molten outer core. At the boundary between the inner and outer core, a.k.a. the Bullen discontinuity, bits of liquid iron and nickel from the outer core interact with material in the inner core and begin to rise.

“This light material is buoyant, and is very likely to be a major driver of circulation within Earth’s liquid outer core,” Williams says. “It is this circulation within Earth’s roiling metallic liquid outer core which generates Earth’s magnetic field.”

That magnetic field is critical for all life on Earth. It’s what protects us (and our electronics) from harmful cosmic and solar radiation. The more we know about how it forms, the better.