The Earth has in its innermost part a solid metal ball of about 2,442 kilometers in diameter, a kind of “planet within a planet”, whose existence influences the terrestrial gravity, magnetic fields and life on the surface, while less as we know it.
How Earth’s inner core formed, grew, and evolved over time remains a mystery. A team of researchers led by the University of Utah is trying to unravel this mystery with the help of seismic waves from natural earthquakes.
The inner core is not the homogeneous mass scientists once assumed, but more like a tapestry of different “fabrics,” according to Guanning Pang, a former doctoral student in the Department of Geology and Geophysics at the University of Utah who dedicated his doctoral thesis to a detailed study of this part of the planet and is now publishing the results in collaboration with an international team of experts.
“For the first time we confirm that this kind of inhomogeneity is everywhere within the inner core,” Pang said. Now a postdoctoral researcher at Cornell University, Guanning Pang is the lead author of a new study, published in the journal Nature, that opens a window into the deepest reaches of Earth.
“Our study was trying to look inside the inner core,” said university seismologist Keith Koper, who supervised the study. “It’s like a border area. Anytime you want to imagine the inside of something, you have to remove the surface effects. So this is the hardest place to image, the deepest part, and there are still things that are unknown about it. “.
This research took advantage of a data set generated by a global seismology data network originally created to detect nuclear explosions. As will be recalled, in 1996, the UN established the Preparatory Commission for the Comprehensive Nuclear Test Ban Treaty Organization, CTBTO, to ensure compliance with the international treaty that prohibits such explosions.
A central piece of the work of this commission is the International Monitoring System (IMS), which features four systems for detecting explosions using advanced detection instruments located around the world. While their purpose is to enforce an international ban on nuclear detonations, they have produced a wealth of data that scientists can use to shed new light on what’s going on inside the Earth, oceans, and atmosphere.
These data have facilitated the investigation of meteorite explosions, identified a colony of pygmy blue whales, forecast the weather, and provided information about how icebergs form.
While the Earth’s surface has been thoroughly mapped and characterized, its interior is much more difficult to study as it is not directly accessible. The best tools for detecting this interior are seismic waves from earthquakes, which propagate from the planet’s thin crust and vibrate through its rocky mantle and metallic core.
“The planet formed from asteroids that were accreting [in space]. They’re colliding with each other and you’re generating a lot of energy. So the whole planet, when it’s forming, is melting,” Koper said. “It’s just that the iron is heavier and you get what we call core formation. The metals sink to the center, and the liquid rock is on the outside, and then it essentially freezes over time. The reason all the metals are down there because they are heavier than rocks.
For the past few years, Koper’s lab has been analyzing sensitive seismic data in the inner core. An earlier study, led by Pang, identified variations between the rotations of Earth and its inner core that may have caused a change in day length between 2001 and 2003.
For decades, it has been known – from indirect studies – that the Earth’s core is composed mainly of iron and some nickel, along with a few other elements. The outer core remains liquid, enveloping the solid inner core.
“It’s like a planet within a planet that has its own rotation and is uncoupled by this great ocean of molten iron,” said Koper, a geology professor who directs the University of Utah Seismographic Stations.
The protective field of magnetic energy that surrounds the Earth is created by convection that occurs within the liquid outer core, which extends above the solid core, recalls this expert. Molten metal rises above the solid inner core, cools as it approaches Earth’s rocky mantle, and sinks. This circulation generates the bands of electrons that surround the planet. Without Earth’s solid inner core, this field would be much weaker and the planet’s surface would be bombarded with radiation and solar winds that would strip away the atmosphere and leave the surface uninhabitable.
For the new study, the team analyzed seismic data recorded by 20 sets of seismometers placed around the world, including two in Antarctica. The closest to Utah is just outside of Pinedale, Wyoming. These instruments are inserted into holes drilled up to 10 meters into granite formations and are arranged in patterns to focus the signals they receive, similar to how satellite dishes work.
Pang analyzed seismic waves from 2,455 earthquakes, all with a magnitude greater than 5.7, or roughly the force of the 2020 quake that struck Salt Lake City. The way these waves bounced off the inner core helps map its internal structure.
Smaller earthquakes do not generate waves strong enough to be useful for study.
“This signal coming back from the inner core is really small. The size is on the order of a nanometer,” Koper said. “What we’re doing is looking for a needle in a haystack. So these baby echoes and reflections are very hard to see.”
Scientists first used seismic waves to determine that the inner core was solid in 1936. Prior to the discovery by Danish seismologist Inge Lehmann, the entire core was assumed to be liquid as it is extremely hot, approaching 10,000 degrees. Fahrenheit, roughly the temperature at the surface of the Sun.
At some point in Earth’s history, the inner core began to “nucleate,” or solidify, under the intense pressures at the center of the planet. It is unknown when that process began, but the Utah team got important clues from seismic data, which revealed a scattering effect associated with waves penetrating deep into the core.
“Our biggest finding is that the inhomogeneity tends to be strongest when you go deeper toward the center of the Earth, when the core tends to be strongest,” Pang said.
“We think this tissue is related to how quickly the inner core grew. A long time ago, the inner core grew very fast. It reached an equilibrium and then started to grow much more slowly,” Koper said. “Not all of the iron turned solid, so some liquid iron could get trapped inside.”
Researchers from the University of Southern California, the University of Nantes in France and Los Alamos National Laboratory participated in the study.
