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Earth has been compared to an onion, with layers of atmosphere, crust, mantle, and interior. If you metaphorically peel off one layer, you may find one underneath with its own unique physical properties. Now, a study published in Nature Communications asserts that even Earth’s inner core might justify the onion analogy.
University of Münster geochemist Carmen Sanchez-Valle led research to explain the odd behavior of seismic waves as they pass through Earth’s inner core. Researchers had observed that earthquake sound waves don’t behave as expected in Earth’s inner core. There, starting about 3,200 miles beneath our feet, they travel 3 to 4 percent faster when they’re parallel to Earth’s rotation axis than when they’re in the equatorial plane.
These deviations in wave behaviors, or “anisotropy” in seismic jargon, get more extreme as waves move deeper through the inner core. A leading hypothesis for anisotropy holds that when seismic waves may encounter organized crystals of iron alloy in Earth’s structured center, the waves literally deform the materials that make up the core.
Read more: “The First Good Glimpse of the Earth’s Mantle”
Previous studies have provided evidence that, although the Earth’s inner core is mostly made of iron, with a small bit of nickel, its density implies some lighter elements in alloy with the iron-nickel crystal structure. Based on experimental work to see which combination of elements yields the estimated density, the researchers synthesized the materials of Earth’s core as a mixture of iron with small amounts of silicon and carbon.
To test the leading hypothesis, they mimicked the extreme conditions at Earth’s core, which required some extremely specialized equipment, appropriately dubbed the Extreme Conditions Science beamline PO2.2. Although it sounds like science fiction, the equipment pressed the alloy samples between two diamond anvils with flattened tips—heating them to about 1500 degrees Fahrenheit and compressing them to about 1 million times atmospheric pressure.
“The diffraction patterns were analyzed after the experiment to derive plastic properties—specifically, yield strength and viscosity—of the iron-silicon-carbon alloys,” to see how readily the alloys deformed, explained co-author and University of Münster geoscientist Efim Kolesnikov in a statement. In modeling how waves would move through the compressed sample versus pure iron, Kolesnikov and colleagues found that the deviant wave behavior could be responding to a gradient in the materials that make up the core.
Deeper into Earth’s core, the percentage of iron is thought to increase, in essence forming layers with different properties, depending on the amounts of silicon and carbon present. Anisotropy might scale with these onion-like layers, concluded Kupenko.
So, the layering of iron alloyed with silicon and/or carbon in Earth’s inner core may explain seismic wave behavior that has intrigued researchers for decades. Our planet is still revealing its surprising complexity after more than 4.5 billion years. 
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Lead image: Johan Swanepoel / Shutterstock
