It turns out that the core of the Earth does move (but that it does so at two speeds)

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Nuclear tests carried out in the 1960s and 1970s have brought us a curious discovery half a century later. A team of researchers from the University of Southern California has found in them evidence that the Earth’s core oscillates, alternating periods in which it rotates faster than the Earth’s surface with periods in which it takes advantage. This phenomenon would explain a curious phenomenon, the six-year cycles observed in the length of the day.


History of discoveries.
To understand the study relevance it is convenient to put in antecedents, two concretely. First, observations in the mid-1990s showed that the Earth’s core was moving faster than the surface. This phase shift is known as superrotation.

Initial calculations put this difference at one degree per year, but later estimates lowered that figure. John Vidale, one of the co-authors of this new study, was involved in these new estimates that confirmed and at the same time qualified the first calculations.

The second key antecedent is the discovery of small oscillations (a matter of milliseconds) in the length of the day, that is, in the time that the earth’s surface needs to complete its rotation. These cycles oscillate every six yearsand that’s it for almost a decade Scientists suspected that this wobble had to do with changes in the rate of rotation of the Earth’s inner layers.

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Superrotation and subrotation.
The study that just publish the magazine Science Advances points in the latter direction. The analysis signed by Vidale and Wei Wang focuses on the period between 1969 and 1974, in which they observed changes in the speed of rotation of the Earth’s core. They found that the core was spinning somewhat more slowly than expected between 1969 and 1971, but that the so-called super-rotation had reversed in the period between 1971 and 1974.

This means that during the indicated time, the terrestrial core moved more slowly than the surface of our planet, that is, it underrotated. The results were in agreement with the data observed at the time on the length of the day, which confirms the link between oscillations in the relative velocity of the core and changes in the length of the day on Earth.

Earthquakes and nuclear bombs.
The researchers used a curious mechanism to carry out their calculations, the analysis of seismic waves caused by Soviet nuclear tests between 1971 and 1974. During that time, the USSR detonated several bombs in the subsoil of the Novaya Zemlya archipelago. The seismic data was collected by the Large Aperture Seismic Array (LASA), a US Air Force seismic event detection system located in Montana, used in the Cold War as a way to detect nuclear explosions.

After obtaining the first results, Wang and Vidale replicated the procedure for the previous three years, taking advantage of two nuclear tests this time on Amchitka Island, near Alaska, in the years 1969 and 71.

A known oscillation.
Combining the seismic data with observations about the length of the day, the researchers confirmed the initial intuition. “The idea that the inner core oscillates corresponds to a model that was already there, but the community [científica] has been divided on whether this was feasible” explains Vidale. “We went into this expecting to see the same direction and rate of rotation in previous atomic tests, but we saw the opposite. We were quite surprised to find that it was moving in the opposite direction.”

The future direction of research.
Expanding the data record is going to be difficult. On the one hand, due to the end of nuclear testing programs and, on the other, due to the closure of facilities such as LASA in Montana. That’s not to say the research is impossible, but it will have to rely on “conventional” seismic waves, those caused by earthquakes. The problem is that the information obtained in this way is less precise.

Analyzing what lies below the surface of planets (and other objects) is a complex task due to the mere fact that we cannot simply uncover and “look” at what is there. However, the authors of this latest study point out that there is still a long way to go if we want to better understand what this kind of race between the core and the surface is like.

“One of the questions we’ve tried to answer is does the inner core progressively move or is it rather fixed compared to everything else in the long run? We are trying to understand how the inner core was formed and how it moves over time,” adds Vidale.

Image | A.Steiwi

Nuclear tests carried out in the 1960s and 1970s have brought us a curious discovery half a century later. A…

Nuclear tests carried out in the 1960s and 1970s have brought us a curious discovery half a century later. A…

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