Bengaluru: Scientists believe that the Earth’s inner core spins faster than the rest of the planet, but new findings suggest that sometime around 2009, it slowed down like something out of a science fiction movie, and now spins slower.
The results, obtained from studying seismic data, seem to indicate that the Earth’s inner core changes its speed of rotation every 60-70 years on an average.
This model of the inner core’s rotation could provide a potential explanation for other phenomena around Earth that oscillate with the same time period, such as variations in the magnetic field around the Earth, global mean sea level, activity in some parts of the mantle (solid bulk of the Earth’s interior), and even global mean temperature.
The findings add to the growing body of work that is attempting to explain the rotation of the inner core.
The study, conducted by researchers Yi Yang and Xiaodong Song from Peking University in Beijing, was published in the journal Nature Geoscience on Monday. Song was also involved in the very first paper which showed evidence of the inner core’s rotation in 1996.
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Layers under our feet
In the new study, the researchers studied quakes from 1995 to 2021, with a focus on doublets or repeating quakes with similar waveforms detected at the same location. By analysing changes in the time and propagation of these signals, they could estimate the rotation of the inner core, which is believed to move independently from the mantle and rest of the planet.
They found that the inner core’s super-rotation (faster rotation relative to the rotation of the mantle) stopped around the year 2009. They said these changes were observed from different parts of the Earth and confirmed a planet-wide phenomenon. Subsequently, the inner core began sub-rotation or rotating slower than the mantle.
The team also discovered a similar pattern in the early 1970s, which suggests that variation in speed occurred around that time as well.
The data agrees with variations in the magnetic field of the Earth as well as rotation, and the researchers say it adds more evidence to the inner core periodicity of 60-70 years on an average.
The Earth’s inner structure consists of four major layers: the outermost crust, the viscous but solid mantle below it, the liquid iron-nickel outer core, and the solid iron inner core.
The inner core is a solid ball that is about 3/4th the size of the moon. It was discovered in 1936 when seismologists noticed patterns in how seismic waves caused by earthquakes travelled through the interior of the planet. Changes in the speed and direction of these waves showed that the inner core must be solid.
This solid ball is encased in the liquid outer core and thus spins freely. The liquid core essentially decouples the inner core from the rest of the planet, allowing it to rotate faster or slower.
The rotation of the outer liquid core produces the magnetic field around the Earth and the rotation of the inner core subsequently occurs due to electromagnetic forces.
When there are variations in the structure of the mantle and outer core, gravitational anomalies drive up or down the speed of the rotation of the inner core.
What else could it be?
Since we do not understand the interior of the Earth fully and cannot directly observe it, we derive knowledge about it from other effects seen around us.
Findings from the 1960s started to suggest that the inner core could go through periods of “super rotation”, while others indicated that this faster spin was par for the course.
Some models have suggested that — according to analysis of seismic waves from nuclear tests in the 1960s and 1970s — the Earth’s inner core rotated slower than the mantle in the 1960s. Scientists believe that from the 1970s onwards, the inner core has been spinning faster than the rest of the planet.
Since these models are built using seismic wave propagation, some scientists also suggest that uneven surface or deformities on the surface of the inner core, or changes at the inner-outer core boundary, could change the propagation of these waves.
The changes in the speed of the inner core do not affect life on the Earth’s surface, but cause variations in other geophysical and climate planetary phenomena. This includes the length of a day as well.
Future models which will build upon more data and seismic observations over time will be able to understand the changes in the rotation of the inner core better over the next few years.
(Edited by Nida Fatima Siddiqui)
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