Seismic waves reveal textures in Earth’s inner core

July 5 () –

Seismic data reveals that Earth’s solid metal inner core, surrounded by another, more fluid shell, displays a variety of acquired textures

The data set was generated over the past 27 years by a network of seismometers created to enforce the nuclear test ban treaty, as published in the journal ‘Nature’.

At the center of the Earth is a solid metal ball, a kind of “planet within a planet”, the existence of which makes life on the surface possible, at least as we know it.

How Earth’s inner core formed, grew and evolved remains a mystery as a team of University of Utah researchers is trying to unravel with the help of seismic waves from natural earthquakes. Although this sphere of 2,442 kilometers in diameter represents less than 1% of the total volume of the Earth, its existence is responsible for the magnetic field of the planet, without which the Earth would be a very different place.

But the inner core is not the homogeneous mass scientists 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, cited by Phys.org.

“For the first time we confirm that this type of inhomogeneity is everywhere within the inner core,” says Pang, now a postdoctoral researcher at Cornell University and lead author of the study, which opens a window into the depths of Earth.

“What our study was about was trying to look inside the inner core,” explains U seismologist Keith Koper, who supervised the study. “It’s like a border area. Anytime you want to get a picture of the inside of something , superficial effects must be eliminated. 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 special data set generated by a worldwide network of seismic networks created to detect nuclear explosions. In 1996, the United Nations created the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) Preparatory Commission to ensure compliance with the international treaty banning such explosions.

Its centerpiece is the International Surveillance System (IVS), which has four blast detection systems using advanced detection instruments located around the world. Although their goal is to enforce the international ban on nuclear detonations, they have provided a wealth of data that scientists can use to shed new light on what is happening inside the Earth, oceans, and atmosphere.

These data have facilitated the investigation of meteorite explosions, the identification of a colony of pygmy blue whales, advanced weather forecasting and understanding 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 hidden realm are seismic waves from earthquakes that propagate from the planet’s thin crust and vibrate through its rocky mantle and metallic core.

“The planet was formed from asteroids that were accumulating in space. They collided with each other and generated a lot of energy. So the whole planet, when it is forming, is melting,” Koper says. “Iron is simply heavier and what we call core formation occurs. The metals sink in the center, and the liquid rock is outside, and then it essentially freezes over time. The reason all the metals are down there It’s because they’re heavier than rocks,” Explain.

For the past few years, the Koper lab has been analyzing sensitive seismic data to the inner core. An earlier study, led by Pang, identified variations between the rotations of Earth and its inner core that could have caused a change in day length between 2001 and 2003.

The Earth’s core, which measures about 6,000 kilometers in diameter, 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,” explains Koper, a professor of geology who directs the University of Utah Seismographic Stations.

The protective field of magnetic energy that surrounds the Earth is created by convection within the liquid outer core, which extends 2,260 kilometers above the solid core, he says. 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 planetary surface would be bombarded by radiation and solar winds that would devastate the atmosphere and make the surface uninhabitable.

For the new study, the U team examined 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 of up to 10 meters in granite formations and are arranged in patterns to concentrate the signals they receive, in a similar way to the operation of satellite dishes.

Pang analyzed seismic waves from 2,455 earthquakes, all of them greater than magnitude 5.7, or roughly the force of the 2020 quake that struck Salt Lake City. The way these waves bounce 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 tiny. It’s on the order of a nanometer in size,” Koper says. “What we’re doing is looking for a needle in a haystack. So these baby echoes and reflections are very difficult. to see”.

Scientists first used seismic waves to determine that the inner core was solid in 1936. Before Danish seismologist Inge Lehmann’s discovery, the entire core was assumed to be liquid, as it is exceedingly hot, nearly 10,000 degrees Fahrenheit. , approximately the surface temperature 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 not yet known when that process began, but the team got important clues from the 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 greater at deeper depths. Toward the center of the Earth it tends to be stronger,” Pang says.

“We think this tissue is related to the growth rate of the inner core. A long time ago, the inner core grew very fast. It reached an equilibrium and then started to grow much slower,” he explains. Not all of the iron solidified, so some of the liquid iron could have been trapped inside.”