The Blue Marble image of Earth captured by the Apollo 17 mission (Credit: NASA/Apollo 17 crew/Public Domain/

Geologists have long maintained that our Earth comprises four layers — the crust, the mantle, the outer core, and, the deepest layer, the inner core. However, a team of scientists led by Jo Stephenson, a doctoral student in seismology at Australian National University in Canberra, now asserts that our planet may be harboring a mysterious, fifth layer — an "inner-inner" core as well.

"We found evidence that may indicate a change in the structure of iron, which suggests perhaps two separate cooling events in Earth's history," Ms. Stephenson said. "The details of this big event are still a bit of a mystery, but we've added another piece of the puzzle when it comes to our knowledge of the Earth's inner core."

The Earth's core, located 1,800 miles (2,897 kilometers) below the surface, consists of two distinct layers. The outer core, which borders the mantle, is made of liquid metals and boasts temperatures as high as 4,000 degrees Fahrenheit (2204 degrees Celsius). The solid inner core — a ball of iron and traces of nickel — is estimated to be 9,800 degrees Fahrenheit (5,430 degrees Celsius), or about the temperature at the surface of the Sun.

Earth’s various layers, including the crust, mantle, outer core, and inner core (Credit: Kelvinsong - Own work/ CC BY-SA 3.0,/

Since conducting fieldwork in these extreme temperatures is impossible, researchers rely on signals from earthquake waves — which change direction and speed depending on the material they pass through — to explore the planet's depths. Over the years, scientists have noticed that the seismic waves exhibit a strange yet consistent behavior whenever they pass through the inner core. The waves moving north to south — parallel the Earth's rotational axis — are always faster than those moving parallel to the Equator. This phenomenon is known as anisotropy.

However, in 2015, scientists at the University of Illinois and Nanjing University in China noticed a discrepancy in this behavior. The anisotropy at the heart of the inner core did not match the rest of the inner core. After further investigation, they found that the iron crystals in the outer layer of the inner core are aligned directionally — north to south. Meanwhile, those in the "inner-inner" core point roughly east to west. The researchers theorized it could be either because the "inner-inner" core was made of a different type of crystal or that the Earth's core was still evolving. However, no further research was conducted.

The Earth's inner core has an inner core of its own, with crystals aligned in a different direction (Credit: Lachina Publishing Service)

To further investigate the theories, Stephenson and her colleagues collected a dataset of about 100,000 records of seismic waves that passed through the center of the core. They then applied an algorithm to determine the best explanation of what was going on in the deepest part. “The idea of another distinct layer was proposed a couple of decades ago, but the data has been very unclear,” Stephenson explains. “We got around this by using a very clever search algorithm to trawl through thousands of the models of the inner core.”

Sure enough, the researchers found that in the heart of the core, the anisotropy of the waves was no longer perfectly parallel to the equator — they deviated by 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.

The scientists, who published their findings in the Journal of Geophysical Research: Solid Earth in late November 2020, are now working with mineral physicists to determine the composition of the "inner-inner" core. They believe that further analysis may help us better understand our planet's formation. “It’s very exciting — and might mean we have to re-write the textbooks!” Stephenson said.