Scientists have discovered a new core at the center of the Earth
Back in my day, there were only four layers of Earth: the crust, mantle, liquid outer core and solid inner core. Now, scientists have revealed a new, distinct layer within our planet’s inner core, which could help inform the evolution of Earth’s magnetic field.
In a new study released this week, a pair of seismologists at the Australian National University documented new evidence of a 400-mile thick solid metallic ball at the center of Earth’s inner core – like the smallest figurine of a massive, planetary Russian nesting doll set. The new layer consists of an iron-nickel alloy, like other parts of the core. But it has a different crystal structure that causes shock waves from earthquakes to reverberate through the layer at different speeds than the surrounding core, the study found.
“Clearly, the innermost inner core has something different from the outer layer,” said Thanh-Son Pham, lead author of the study. “We think that the way the atoms are [packed] in these two regions are a slightly different.”
Researchers study the inner core to better understand Earth’s magnetic field, which protects us from harmful radiation in space and helps make life possible on our home planet. Geophysicists surmise the inner core could have formed less than a billion years ago, which is relatively young on a geologic time scale. The study authors explain the inner core grows outward by solidifying materials from the liquid outer core, releasing heat and creating convection currents. This convection generates Earth’s magnetic field.
The inner core, which was discovered in 1936 by Danish seismologist Inge Lehmann, makes up less than 1 percent of Earth’s volume (Earth’s center is located about 4,000 miles below the surface). Its distance beneath the surface and small size, though, make it difficult for scientists to measure by direct measurements, so instead they study shock waves triggered by earthquakes.
When a large earthquake strikes, the resulting shock waves, or seismic waves, can bounce back and forth from one side of the Earth to the other like a ping-pong ball, said Pham. Seismic waves travel at different speeds through Earth’s different layers depending on its density, temperature and composition. Like a radiologist studying a patient’s internal organs, scientists use instruments known as seismometers around the world to measure these oscillations and learn about the inner workings of our Earth.
Twenty years ago, researchers used seismograph data and proposed the existence of a fifth layer. Since then, Pham said the evidence of the innermost inner core “has become strengthened with time with more and more data.” But his new study takes it further, analyzing unprecedented seismograph data.
“The breakthrough in this study is that we find a new way to sample the very center of the Earth’s inner core,” said Pham. The team, he said, has even more evidence to prove the “innermost inner core actually exists.”
In the new study, the team observed several earthquakes traversing the diameter of Earth – sometimes up to five times – something that researchers had not recorded “in the history of seismology,” said Pham, who noted previous studies documented only a single bounce. They found that the seismic waves passed through the innermost inner core at different speeds than through the surrounding core, depending on the direction of the wave.
Specifically, waves passing through the innermost inner core slowed down when approaching from an oblique angle to the equator. Meanwhile, waves passing through the outer inner core slowed down when passing through along the equator.
Pham said the speed likely differs based on the wave direction due to a physical property known as anisotropy, which allows a material to possess different properties in different directions. We commonly see anisotropy in wood, which is easier to chop along its grain than against it.
The uniqueness of this innermost core is subtle and not as sharp as other layers, Pham admitted. For instance, if you traveled from the mantle to the outer core, you would go from a mostly solid to liquid and experience different chemical compositions. But if you traveled from the inner core to the innermost core, you would see a transition in crystal structure but the same iron-nickel alloy.
Geophysicist John Tarduno, who was not involved in the research, said the idea of an innermost core has been proposed before but this new data substantially strengthens the case that “there is actually an innermost inner core with a different structure from the outermost inner core.”
“The very existence of this innermost inner core makes us think about how it could form,” said Tarduno, who is a professor at geophysics at the University of Rochester. The study authors said the formation of the innermost inner core could be evidence of “a significant global event from the past” that spurred a change in Earth’s inner core.
Tarduno, who researches how the inner core may have formed, has his own idea. His research suggests the formation of this innermost inner core could be tied to a change in plate tectonics hundreds of millions of years ago. He posits that thick, aged slabs of oceanic crust sank until they piled up at the base of the mantle, affecting how heat flowed out of the core. This subsequently changed how the inner core grew.
Tarduno said this “the new analysis is exciting because it bolsters the case” for his plate tectonic mechanism.
“What we could be looking at in this innermost inner core is actually a signal of a change of the plate tectonics regime,” said Tarduno, who published his findings last year.
Both Tarduno and Pham said learning the origins of the inner core layers can help us understand more about how the magnetic field formed – and, by extension, how life can survive on Earth and other planets.
“The formation of the inner core was extremely important for creating a long term habitable planet because the inner core powered the magnetic field, that powered magnetic shielding,” said Tarduno. “Without that, we would have gradually lost water from the planet.”
Learning more about how inner core “in turn can help teach us more about how other planets might or might not be habitable.”