What’s Inside Earth’s Inner Core? A Shifting, Spinning Mystery’s Latest Twist

The Earth’s core is a solid metallic sphere situated at the center of the planet, acting as a “planet within a planet.” Its presence is crucial for sustaining life on the surface in the way we know it.

The origins, growth, and evolution of Earth’s inner core have remained a mystery, but a team of researchers led by the University of Utah is determined to unravel its secrets using seismic waves generated by natural earthquakes.

Although this sphere, with a diameter of 2,442 kilometers, makes up less than 1% of the Earth’s total volume, its existence is responsible for the planet’s magnetic field, which plays a vital role in shaping Earth’s characteristics.

Contrary to previous assumptions, the inner core is not a homogeneous mass. Instead, it resembles a tapestry of different materials or “fabric,” as described by Guanning Pang, a former PhD student in the Department of Geology and Geophysics at the University of Utah.

“For the first time we confirmed that this kind of inhomogeneity is everywhere inside the inner core,” Pang remarked.

Currently working as a post-doctoral researcher at Cornell University, Pang is the lead author of the new study published today in the journal Nature, offering unprecedented insights into the deepest realms of our planet. He conducted this study during his doctoral research at the University of Utah.

Exploring the depths

Keith Koper, a seismologist at the University of Utah who supervised the study, explained the focus of their research: “What our study was about was trying to look inside the inner core. It’s like a frontier area. Anytime you want to image the interior of something, you have to strip away the shallow effects. So this is the hardest place to make images, the deepest part, and there are still things that are unknown about it.”

This research took advantage of a unique dataset generated by a global network of seismic arrays designed to detect nuclear explosions. In 1996, the Comprehensive Nuclear-Test-Ban Treaty Organization’s Preparatory Commission (CTBTO) was established by the United Nations to enforce compliance with an international ban on nuclear detonations.

The centerpiece of the CTBTO is the International Monitoring System (IMS), consisting of four systems equipped with advanced sensing instruments placed strategically worldwide to detect explosions. While their primary purpose is to ensure the ban on nuclear tests, these systems have generated vast amounts of data that scientists can utilize to gain new insights into Earth’s interior, oceans, and atmosphere.

This data has contributed to various research endeavors, such as enhancing our understanding of meteor impacts, identifying colonies of pygmy blue whales, advancing weather forecasting capabilities, and shedding light on the formation of icebergs.

While the Earth’s surface has been extensively mapped and characterized, exploring its interior is significantly more challenging due to the inability to directly access it. The most effective means of investigating this hidden realm is by studying seismic waves generated by earthquakes, which propagate through the planet’s thin crust, resonate through the rocky mantle, and penetrate the metallic core.

“The planet formed from asteroids that were sort of accreting [in space]. They’re running into each other and you generate a lot of energy. So the whole planet, when it’s forming up, is melting,” explained Koper.

“It’s simply that the iron is heavier and you get what we call core formation. The metals sink to the middle, and the liquid rock is outside, and then it essentially freezes over time. The reason all the metals are down there is because they’re heavier than the rocks.”

“A planet within a planet“

Over the past few years, Koper’s laboratory has been closely examining seismic data that is sensitive to the Earth’s inner core. In a previous study led by Pang, variations in the rotation of the Earth and its inner core were identified, which may have caused a shift in the length of the day between 2001 and 2003.

The Earth’s core, spanning approximately 4,300 miles in diameter, consists primarily of iron and some nickel, along with a few other elements. The outer core remains in a liquid state, enveloping the solid inner core.

“It’s like a planet within a planet that has its own rotation and it’s decoupled by this big ocean of molten iron,” explained Koper, a professor of geology and the director of the U of U Seismograph Stations (UUSS).

The protective magnetic field surrounding the Earth is generated by convection processes occurring within the liquid outer core, which extends 2,260 kilometers (1,795 miles) above the solid core. Molten metal rises above the solid inner core, cools as it nears the Earth’s rocky mantle, and then sinks. This circulation creates the bands of electrons encircling the planet. Without the Earth’s solid inner core, this magnetic field would be significantly weaker, exposing the planetary surface to radiation and solar winds that would strip away the atmosphere and render the surface uninhabitable.

For their recent study, the University of Utah team examined seismic data recorded by 20 arrays of seismometers positioned worldwide, including two in Antarctica. The closest seismometer to Utah is located outside Pinedale, Wyoming. These instruments are inserted into boreholes drilled up to 10 meters deep in granite formations and arranged in specific patterns to concentrate the received signals, similar to the functioning of parabolic antennas.

Pang analyzed seismic waves from 2,455 earthquakes, all with magnitudes exceeding 5.7, which is comparable to the strength of the 2020 earthquake that shook Salt Lake City. By studying how these waves interacted with the inner core, they were able to map its internal structure.

Smaller earthquakes do not generate waves strong enough to be useful for this study.

“This signal that comes back from the inner core is really tiny. The size is about on the order of a nanometer,” Koper added.  “What we’re doing is looking for a needle in a haystack. So these baby echoes and reflections are very hard to see.”

The changing core

Seismic waves were first employed in 1936 to determine the solid nature of the Earth’s inner core. Prior to this discovery by Danish seismologist Inge Lehmann, it was believed that the entire core was liquid due to its extremely high temperature, approaching 10,000 degrees Fahrenheit, which is similar to the temperature on the surface of the sun.

At some point in Earth’s history, the inner core began the process of nucleation or solidification under the immense pressures found at the planet’s core. The exact time when this process commenced remains unknown, but the research team at the University of Utah gained important insights from the seismic data. They observed a scattering effect associated with waves penetrating the core’s interior.

“Our biggest discovery is the inhomogeneity tends to be stronger when you get deeper. Toward the center of the Earth it tends to be stronger,” Pang noted.

“We think that this fabric is related to how fast the inner core was growing. A long time ago the inner core grew really fast. It reached an equilibrium, and then it started to grow much more slowly,” Koper explained. “Not all of the iron became solid, so some liquid iron could be trapped inside.”

Image Credit: Shutterstock