The Discovery of a Complex Inner Core

New research published in Nature Communications suggests that Earth’s inner core may be more complex than previously thought, with two distinct layers. The discovery was made using a previously undescribed seismic wave that not only travels through the core, but also bounces back and forth, collecting valuable data about the core’s structure along the way.

Researchers focused on earthquakes of magnitude 6 or larger that struck in the last decade and combined data on these quakes that were collected at seismic stations around the world. Combining these signals made it possible to detect even very faint reflections of the seismic waves. Of the 200 or so quakes analyzed, 16 events spawned seismic waves that detectably bounced through the inner core multiple times.

The inner core, about 6,600 kilometers across, consists of two main parts: a liquid outer core and a solid inner core. As iron-rich fluid circulates in the outer core, some of the material cools and crystallizes, sinking to form a solid center. That interplay generates Earth’s magnetic field. The solid inner core is thought to be a ball of nickel and iron, but this new research suggests it has two distinct layers: a distinct central region nestled within an outer shell.

Implications for Earth’s Magnetic Field

The exact source of the wave slowdown isn’t clear, but it may be related to the structure of the iron crystals, which may be packed together differently farther into the center. Or it could be from a different crystal alignment caused by some long-ago global event that changed how inner core crystals solidified out of the outer core. Lighter elements present in small amounts in the core — hydrogen, carbon, oxygen — may flow around the solid iron in a liquidlike “superionic” state, further complicating the seismic picture.

The discovery of a hidden heart within Earth’s core has important implications for understanding the planet’s magnetic field, which shields the Earth from charged particles ejected by the sun and helps keep the planet’s denizens safe from too much radiation. “Understanding how the magnetic field evolves is extremely important for the life on Earth’s surface,” says Hrvoje Tkalčić, a seismologist at the Australian National University in Canberra.

The researchers estimate that this inner heart is roughly 600 kilometers across, or about half the diameter of the full inner core. And the pair was able to assess the direction of the slowest waves at about 50 degrees relative to the Earth’s rotation axis, providing more insight into the region.

While this discovery sheds new light on Earth’s inner core, seismologist Paul Richards of Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y. cautions that the team’s interpretation of the inner core’s structure from those waves “is probably more iffy.” As the waves bounce back and forth, they can become weaker and more difficult to see in the data. Many further observations will be needed to decide what these new data can reveal about the heart of the planet.