A new research has given the first accurate estimate of how much faster the Earth’s inner core is rotating compared to the rest of the planet.
Previous research had shown that the core was rotating faster than the rest of the planet. However, scientists from the University of Cambridge have discovered that earlier estimates of 1 degree every year were inaccurate and that the core is actually moving much slower than previously believed -- approximately 1 degree every million years. The inner core grows very slowly over time as material from the fluid outer core solidifies onto its surface. During this process, an east-west hemispherical difference in velocity is frozen into the structure of the inner core.
"The faster rotation rates are incompatible with the observed hemispheres in the inner core because it would not allow enough time for the differences to freeze into the structure,” said Lauren Waszek, a research student from the University of Cambridge’s Department of Earth Sciences and first author on the paper. "This has previously been a major problem, as the two properties cannot coexist. However, we derived the rotation rates from the evolution of the hemispherical structure, and thus our study is the first in which the hemispheres and rotation are inherently compatible,” he added.
The scientists used seismic body waves that pass through the inner core -- 5,200km beneath the surface of the Earth -- and compared their travel time to waves, which reflect from the inner core surface. The difference between the travel times of these waves provided them with the velocity structure of the uppermost 90 km of the inner core.
They then had to reconcile this information with the differences in velocity for the east and west hemispheres of the inner core. First, they observed the east and west hemispherical differences in velocity. They then constrained the two boundaries that separate the hemispheres and found that they both shifted consistently eastward with depth. Because the inner core grows over time the deeper structure is therefore older, and the shift in the boundaries between the two hemispheres results in the inner core rotating with time. The rotation rate is therefore calculated from the shift of the boundaries and the growth rate of the inner core.
"This result is the first observation of such a slow inner core rotation rate,” said Waszek "It therefore provides a confirmed value which can now be used in simulations to model the convection of the Earth’s fluid outer core, giving us additional insight into the evolution of our magnetic field,” he added.
The findings are published in the journal Nature Geoscience.