Growing, turning… what is happening in the Earth’s core?

What’s going on with the inner, solid part of the Earth? This inner core, which consists mainly of iron and is about the size of Pluto with a radius of 1,220 km, rotates at varying speeds relative to the layers around it, the outer core and the mantle, according to two Chinese geophysicists. From the 1960s, the inner core rotated faster. But around 2009 that difference suddenly disappeared, so they calculated on January 23 Nature Geoscience.

But other scientists have their doubts. “We know that something changes in the inner core over time. But interpretations vary widely about what it is exactly,” says Arwen Deuss, professor of structure and composition of the deep earth at Utrecht University. Even more emphatic is Professor Lianxing Wen, affiliated with Stony Brook University in New York and the University of Science and Technology of China in Hefei. “The study misinterprets the seismic signals.”

What is it about? Background is the functioning of planet earth. What is its composition and what processes take place in the interior? For example, how does that affect plate tectonics and the Earth’s magnetic field? For answers, science relies heavily on seismology – drilling and sampling rock cannot go deeper than about 12 kilometres.

Seismology registers and interprets the typical waves produced by an earthquake. They propagate in all directions through the earth, and then they are registered at seismic stations. There are different types of waves, with different behavior. For example, the so-called S-waves cannot pass through liquid material. That is why the idea arose at the beginning of the last century that the entire Earth’s core should be liquid. Because S waves that left the earth after an earthquake on one side formed a kind of shadow on the other side, an area where they did not arrive. It was deduced that the S-waves did not pass through the Earth’s interior. So it had to be liquid.

But in 1936 Danish seismologist Inge Lehmann corrected that image. After an earthquake in New Zealand in 1929, she analyzed data from four seismic stations on the other side of the world, including Greenland and Sweden. Between the data, she picked up remarkably weak signals in some places in the cast shadow. She could not explain it there if the interior of the earth were homogeneously liquid.

First through the liquid part

Lehmann focused on P waves, which can propagate through all materials. She reasoned that the P-waves first pass through the liquid part of the core, but then hit a solid part. There they are partly reflected. They lose energy in the process. That, she said, partly explained the weaker signal she observed. Lehmann further reasoned that a P wave can be converted into an S wave at the transition from the liquid to the solid core. The S wave travels through the solid core, then returns to the liquid outer core and changes back into a P wave. Energy is also lost during these transitions. That also resulted in weaker signals. In her 1936 publication, Lehmann very cautiously suggested the possible existence of a solid inner core.

Meanwhile, geophysical research had made things clearer. For example, that the inner core grows

“Seismologists have been looking for decades to prove it,” says Deuss. Yourself published them in 2000in the Geophysical Journal International, the first evidence for the existence of S waves traveling through the inner core. “It wasn’t that important back then Science or Nature to come.”

Meanwhile, geophysical research had made things clearer. For example, that the inner core grows. Deuss: “By about a millimeter a year.” This growth explains the existence of the Earth’s magnetic field. The iron-nickel alloy crystallizes at the boundary of the inner and outer core, releasing heat. That heat drives currents in the liquid outer core, like in a lava lamp. These convection currents are believed to generate the Earth’s magnetic field. This field protects the Earth from cosmic rays.

Seismic station in Alaska

Based on seismic research, two scientists concluded in 1996 that the inner core rotated independently of, and faster than, the outer core. They examined data from earthquakes near the South Sandwich Islands, in the South Atlantic Ocean. The waves of the quakes had arrived at the seismic station in College, Alaska, among other places. There were several decades of data.

The two seismologists compared the arrival times of two types of waves: waves that travel through the outer core but not the inner core, and waves that travel through both the outer and inner core. They compared data from 1967 and 1982. What turned out? In 1982, the waves that traveled through both the outer and inner core arrived at College 0.4 seconds earlier than in 1967, relative to the waves that traveled only through the outer core.

With that, wrote the authors in Naturethey had provided “evidence” that the inner core moves, “by some type of rotation”. The first author of that article was Xiaodong Song, now professor of geophysics at the University of Illinois. He is also co-author of the recently in Nature Geoscience published paper, in which the researchers show that the rotational speed of the inner core varies.

Gradually more and more processes come into view

But not everyone is equally convinced of that evidence. “There was a lot of criticism of that study from 1996, because the data can also be interpreted differently,” says Deuss. According to her, there are, still, many different ideas about that rotation in circulation. In 1996, Song and co-author Paul Richards concluded that the inner core rotates 1 degree faster annually than the outer core and mantle. Other researchers then arrived at less, or much more. “Sometimes as much as 5 degrees,” says Deuss. She herself assumes a much slower rotation: 0.1 degrees per million years. That’s because she thinks the data shows something completely different. Namely that the composition of the inner core is not the same everywhere. Depending on where the earthquake occurs and where you receive the seismic waves, this can also lead to differences in arrival time. But that means a much slower rotation.

And Lianxing Wen has a completely different explanation. He doesn’t believe in rotation. He thinks that processes at the boundary of the inner and outer core cause variations in the seismic data. “Due to local, temporary differences in temperature, for example, the material at the interface changes.” That would affect the seismic waves that travel through such an area. Another option, he says, is for the inner core to deform temporarily at the interface due to a force acting on it. “The Earth’s magnetic field could be such a force.”

Cold spots in the mantle

As a result, more and more processes are gradually becoming visible. There are also indications that tectonic plates that subduct deep into the mantle cause local relatively cold spots at the interface between the mantle and the outer core. Like deep under Indonesia and Japan. And under the west coast of North and South America. Deuss calls them “cemeteries of cold material.” They cause temperature differences deep in the mantle. These would probably have an effect on the geomagnetic field and the current in the outer core. “And thus also on the crystallization in the inner core.” But whether and how this happens is still unclear.

“There is not yet a model that links and explains all the data,” says John Vidale, professor of geophysics at the University of Southern California. Vidale himself has examined seismic data from nuclear tests in the 1960s. He says “surely” that the inner core rotates faster. And that that speed varies over time. This happens in a recurring 6-year pattern, as he described last year in Science Advances. But Song and his colleague elaborate in their recent publication Nature Geoscience based on a 70-year pattern. “Still others assume a 20-year pattern,” says Vidale.

More data is needed to clarify all this, says Vidale. “Hopefully we can figure it out in the next five to 10 years.”