Scientific Method / Science & Exploration

Earth’s core has its own pole (and it’s at the equator)

New research suggests that the inner core of the Earth has an internal flow …

Modern research techniques allow us to not only study the surface of Earth (and the heavens above it) but also what lies in its depths. Jules Verne wrote about an area alive with mystery and wonder over 146 years ago; modern science indicates that the center of the Earth is a young dynamo powered by the complex interplay of heat transfer, thermodynamics, and fluid mechanics. By accumulating data gathered by bouncing waves off the Earth's core, a team of French researchers have put forth a new theory as to why one hemisphere of the core appears different from the other.

There is a correlation between the seismic wave velocity and attenuation as waves pass through and bounce off the Earth's core. This correlation, to date, cannot be explained in terms of the commonly used viscoelastic attenuation models that are used to represent the Earth's core. When the researchers took the data from a large number of seismic waves and analyzed it in terms of spherical harmonics, they found a dominant signal with a pole (in the sense of the North Pole) close to the equator at a longitude of approximately 80°W. There was an antipole on the other side of the planet.

To explain this, the researchers modeled the inner core in terms of a collection of anisotropic iron grains. Their model predicts a distribution of grain sizes, with larger grains at the antipole (7-15 km) than at the identified pole (300-700m). The authors theorize that convection (movement due to thermal differences) plays a key role in the creation of these differences. 

They suggest there's a uniform velocity flow within the inner core, with a return flow via the outer core. These flows are caused, and balanced, by the crystallization and melting of iron near the two poles they've identified (melting on one side, crystallization on the other). The distribution of crystal grain sizes suggest that the velocity of the flow is parallel to the equator and travels in an West to East orientation. 

The fact that we can observe this process implies that the inner core is still rather young (in a relative sense), less than one or two gigayears. If the rotation were not present, the outward growth of the inner core would result in uniform grain sizes in all spherical directions.

ScienceExpress, 2010. DOI: 10.1126/science.1186212 (About DOIs). 

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