A new research gives the first accurate estimate of how much faster earth’s core is rotating compared to the rest of the planet. Earlier research had shown the earth’s core rotates faster than the rest of the planet. However, estimates of one degree quicker each year were inaccurate as the core is actually moving much slower, approximately one degree every million years, a University of Cambridge study discovered.
Their findings have been published in the journal Nature Geoscience. The inner core grows 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 university said in a statement.
“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,” Lauren Waszek, first author on the paper and Ph.D. student from the University’s department of earth sciences, said.
“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.” For the research, the scientists used seismic body waves which pass through the inner core — 5,200 km 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 which 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. —PTI