New analyses suggest that the Earth’s inner core oscillates, which could explain fluctuations in the planet’s rotation and day length. Despite its importance in providing the heat for all geological processes and being responsible for the Earth’s protective magnetic field, little is known about the inner core. Scientists have been unable to determine whether it has a substructure, how much heat it emits to the upper layers, and when it solidified. The relationship between the movement of the inner core and the rest of the solid Earth is also unclear. Some geological models suggest a stable superrotation, where the mantle and crust rotate slightly slower than the solid core, but the speed of the solid core’s movement is disputed.
The properties of the Earth’s core can only be determined through seismic waves reflected from it, but the variability of earthquake locations and strengths makes it difficult to obtain meaningful data. Researchers at the University of Southern California have used an alternative method, using seismic waves from atomic bomb tests instead of earthquakes. By comparing the seismic data from almost identical atomic bomb tests conducted by the Soviet Union in 1971 and 1974, they found that the inner core moves 0.1 degrees faster than the rest of the solid Earth per year.
The researchers then used the same method to analyze data from two more atomic bomb tests conducted on Amchitka Island in Alaska in 1969 and 1971. They expected to see the same superrotation as in the earlier tests, but instead found that the inner core moved in the opposite direction, 0.05 degrees slower than the rest of the solid Earth per year. This observation suggests that the rotation of the inner core is subject to significant fluctuations, which could explain the slight variations in the Earth’s rotation and day length. However, the data cannot determine whether the inner core only changes its rotation speed or whether it also oscillates slightly in the liquid iron bath of the outer core.
