Earth science: Earth’s inner core rotation may be slowing down

The rotation of the Earth’s solid inner core relative to its mantle may have recently slowed down and could be rotating more slowly than the mantle, according to a study published in Nature Geoscience this week. These findings indicate that changes in this differential rotation could occur on a decadal scale and could aid our understanding of how processes deep in the Earth affect its surface.

The Earth’s inner core is separated from the rest of the solid Earth by the liquid outer core, enabling it to rotate differently from the rotation of the Earth itself. The spin of the inner core is driven by the magnetic field generated in the outer core and balanced by the gravitational effects of the mantle. Knowing how the inner core rotates could illuminate how these layers interact. However, the speed of this rotation, and whether it varies, is debated.

Yi Yang and Xiaodong Song analysed the difference in the waveform and travel time of seismic waves from near-identical earthquakes that have passed through the Earth’s inner core along similar paths since the 1960s. They found that since around 2009, paths that previously showed significant temporal variation have exhibited little change, suggesting that this differential inner core rotation has paused. They also identified that this may be associated with an overall slowing down of the inner core rotation as part of a seven-decade oscillation with a previous turning point occurring in the early 1970s. The authors indicate that this variation correlates with changes in geophysical observations at the Earth’s surface, such as the magnetic field and the length of day.

The authors conclude that this oscillation in differential rotation of the inner core, coinciding with periodic changes in the Earth’s surface system, demonstrates the interaction between different layers of the Earth.