Planetary Core Moves Slower than Previous Thought

If you thought all you needed to know about the Earth’s core was found in the movie “The Core,” then I suggest that it is time you hit Wikipedia for some quick study. In the meantime, scientists at the University of Cambridge have determined that, contrary to previous research, the Earth’s core is actually moving at approximately 1 degree every million years.

Previous research had suggested that the Earth’s core was moving at 1 degree every year.

The structure of the Earth is curst, upper mantle, mantle, outer core, and then inner core. It is that inner core that Cambridge scientists have been focusing on.

The inner core grows slowly, over time, as material from the fluidic outer core solidifies onto the inner cores surface. During this lengthy 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, first author on the paper and a PhD student from the University of Cambridge’s Department of Earth Sciences. “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.”

Studying the inner core is an important addition to the many studies of our planet. As the inner core grows, so too does the heat released from the solidification which drives convection in the fluid in the outer core. This process generates the Earth’s magnetic field, without which we would all be fried by the solar radiation which radiates away from our sun, making life on Earth impossible.

“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.”

The Science

The science behind the results are described in the press release from the University of Cambridge scientists, which I will crop below for fear of getting the science in any way wrong.

For the research, the scientists used seismic body waves which pass through the inner core – 5200km 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.

Source: University of Cambridge
Image Source: Surachit

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