For decades, textbooks have taught us that the Earth is like a Russian doll containing four main layers, one inside the other: crust, mantle, outer core, and inner core. Now, the study of more than 200 earthquakes in the last decade confirms that the inner core contains an even more internal fifth layer: a sphere of almost pure iron with a radius of 650 kilometers, more or less the distance between Madrid and Madrid by car. and Cadiz.
The inner core of our planet would be unrecognizable to any earthling. The pressure is three million times greater than on the surface and the temperature is around 5,500 degrees. “If we could dismantle the planet by removing the mantle and the liquid outer core, we would see that the inner core shines like a star,” explains Hrvoje Tkalčić, a geophysicist at the Australian National University and author of the study on the innermost layer of the earth, posted today in Nature Communications. Understanding the interior of the core is essential to knowing how our planet formed and when it will cease to be a habitable place when the outer core completely solidifies and the magnetic field that protects it from space radiation disappears. That won’t happen for billions of years, but it’s also interesting to understand planets very similar to Earth that have lost their magnetic shield, like Mars.
In 1936, Inge Lehmann, a Danish pioneer in seismology, discovered the Earth’s inner core by studying the propagation speed of earthquakes that traversed the planet from side to side, passing through the core once. Almost 90 years later, the same principle, with much more advanced technology, has made it possible to identify 16 earthquakes among the 200 analyzed that crossed the inner core, not once, but two, three, four and even five times. These are a type of signals that were in the files, but that no one had analyzed before. These seismic waves make it possible to study the composition of the inner core with an “unprecedented” level of detail, the researchers highlight.
All these rebounds were recorded in hundreds of seismic stations spread over all the continents. The data shows that the waves do not propagate uniformly. Those that cross the innermost part of the nucleus travel a few seconds faster if they go parallel to the Earth’s axis of rotation than if they go in a 50 degree plane, for example. However, in the outermost part of the nucleus the propagation is also faster parallel to the axis of rotation, but the slow direction is exactly in the plane of the equator —90 degrees.
This X-ray of Earth shows that the inner core is not a uniform sphere with a radius of 1,221 kilometers, but that inside it there is another even more internal and compact sphere with a radius of 650 kilometers, according to the work of Tkalčić. The study confirms previous estimates, made since the early 2000s. The observations are important to understand how the innermost core formed and how it has progressed in each era of the planet.
“Currently the inner core grows one millimeter a year,” explains Tkalčić. “If this speed has always been constant, it would assume that it is 1,000 million years old. But probably the speed has not been the same at different times on Earth, so we currently have a range of between 200 million years and 1,500 million years; one of the greatest uncertainties of modern science. Perhaps this new study on the border between the two layers of the inner core can improve the models on its evolution”, he highlights.
The study may clear up another difficult puzzle to solve. The iron in the heart of the planet bears little resemblance to that on the surface. The temperatures and pressure are so high that this element forms polyhedral “crystals”. There are two schools, one that maintains that the crystals in the nucleus have the shape of a cube and another that argues that in the innermost part, physics only makes the hexagon possible. The sense of arrangement of these crystals and the atoms that form them would determine the fast and slow directions. “This work seems to lean towards the cubic configuration in the innermost part of the nucleus and the hexagonal one in more external areas,” explains Maurizio Mattesini, professor of Earth Physics and researcher at the Institute of Geosciences (CSIC-UCM), a specialist in the structure. interior of the Earth. “There is still a lack of data to reach a conclusion, but thanks to this type of study, the laboratory simulations and the theoretical calculations of quantum mechanics that we do in my group, for example, they are restricting the possibilities. I think we are close to knowing the truth,” he adds.
The researchers filtered the waves from those 16 earthquakes that passed through the heart of the planet to keep only the interesting part. Then they compared what was observed with different internal core models and the most probable conclusion was the one they present. “When I saw these seismic signals, I was stunned,” admits Puy Ayarza, director of the Department of Geology at the University of Salamanca. “These are completely new seismic phases that allow us to study the inner core very well. It is a very solid piece of work, as there are very few confounding factors”, she highlights.
This work comes a few weeks after another team led by Xiadong Song showed that Earth’s outer core—the liquid part that surrounds the inner core and is key to generating the magnetic field—has slowed down. “This work adds interesting data,” says Song, who works at Peking University’s Institute of Theoretical and Applied Geophysics. “The model they present is similar to the one our group proposed a few years ago. But it is necessary to continue studying to understand the true nature of the deepest part of the planet”, he adds.
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