8:24:27 
2023-01-26 
145
The solid metal core of Earth doesn't rotate perfectly in time with the rest of the planet, and new research suggests that it may have reversed direction.
Seismic waves passing through the Earth have revealed that the inner core of our planet may have stopped rotating over the past decade, and its rotation could be in the process of reversing.
At the centre of our planet is a solid mass of nickel and iron, crushed down by gravity into a sphere roughly 2,400 kilometres wide. This solid core is surrounded by an outer layer of liquid iron and nickel, which generates Earth's protective geomagnetic field, and beyond that are the layers of the mantle and finally the crust.
We know that the interior of Earth is arranged in this way due to decades of study by Earth scientists. They did so by examining the behaviour and timing of seismic waves travelling through the planet in the wake of earthquakes. While most seismic waves stop at the outer core, as they are absorbed by the liquid metal, waves from particularly powerful earthquakes can penetrate right through to the solid inner core and back out.
By carefully selecting specific seismic waves, scientists can focus on studying different properties of the inner core. Repeating earthquakes — those that occur in the same location at different times — are particularly useful for this. Since the seismic waves originate from the exact same location, they should travel along exact same paths and thus have the exact same shape when they're recorded by seismographs. If there's any difference, it's because something inside the Earth changed.
Seismic waves from powerful repeating earthquakes do show up differently at different times. The best interpretation of this is that the rotation of the core is changing over time, relative to the rotation of the mantle. That is... the core is always rotating in the same direction that the entire planet rotates (eastward), but sometimes it rotates faster than the outer layers, other times it rotates slower, and there are times when the rotations match up.
To someone standing in one spot on Earth's surface, these three states can appear, respectively, as though the core is spinning forwards, backwards, or that the spin has stopped.
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