As it has been news during the last few days, a team of Chinese seismologists, formed by Yi Yang and Xiaodong Song, published an article in the scientific magazine Nature Geoscience the 23 January 2023, where they announced that the core of the Earth has slowed down, and that it now turns at the opposite direction. According to the two researchers, this change in the speed of the core of the Earth follows a cycle of seven decades, that to say, it changes direction every 35 years.
The structure of the Earth
The Earth, which has a radius of 6,400 kilometres, is formed by different layers. The principal layers are the crust, the mantle and the core. The core is divided into two other parts: the inner core and the outer core.
The outer core is made out of metals in a liquid state. The inner core, formed almost exclusively by iron, is solid, and has a similar temperature to the surface of the Sun.
Being surrounded by a liquid part, the inner core can turn independently from the rest of the Earth.
Analysing the movements of the inner core
As it is impossible to excavate a tunnel to the centre of the Earth (the maximum depth we have reached has been 12 kilometres), the way to study what happens inside the Earth is to analyse earthquakes. The variation of seismic waves as they go through the planet shows us the composition of the core and its rotation speed. In 2005, Song demonstrated that the inner core of the Earth had a higher rotation speed than the one of the crust, and he detailed that the core makes one more spin than the rest of the planet every 900 spins of the latter.
After having analysed almost 200 earthquakes with computer models that reconstruct the Earth, in 2009 they arrived at the conclusion that the nucleus of the Earth had come to a halt. Since then, the inner core rotates more slowly that the Earth’s crust.
This last appreciation has an incredible implication. Seen from space, now the core of the Earth spins a little more slowly than it did in the past. However, from the perspective of the Earth’s surface, the core spins in the opposite direction. We put an example to understand it better: when you are passing a car in the motorway, from your perspective the other car is moving backwards, but for an outside observer, it is only moving more slowly than you are.
So, the only change that suffers the inner core of the Earth is its rotation speed, and not the direction of rotation. What happens is that from our vantage point, we interpret that it spins in the opposite direction.
A cycle of 70 years
The scientists responsible for this study have also detected that at the end of the 1970s, there was a similar stop in the core of the Earth, for which they inner that the rotation speed of the inner core follows a cycle of about seven decades. This means that the inner core increases and decreases its speed every 35 years, which suggests that there is a resonance that connects all the layers of the Earth and that produced these changes in the inner core.