The Mysterious Inner Core

Three-dimensional distribution of anisotropic fabric within the outer part of the inner core. The rods indicate the directions along which P-waves travel fastest, with the length of the rod indicating the contrast between fast and slow velocities at each point. The outermost inner core has weak anisotropy. There is a puzzling difference between eastern and western hemispheres of the inner core. The central region of the inner core (orange ball) has a distinct anisotropic fabric that is not illustrated. (Image courtesy of X. Song.)
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Seismology reveals that Earth’s inner core is surprisingly complex. Although small (about the size of Earth’s moon), the inner core plays an important role: its progressive freezing generates compositional buoyancy by expulsion of light alloy components into the liquid outer core, which serves as an energy source for the outer core convection that maintains Earth’s magnetic field. In the past two decades, seismic analyses have revealed variations in elastic properties of the inner core both radially and laterally, including multiscale variations in attenuation and anisotropy. To first order, the inner core has an overall anisotropic structure, such that waves travel faster and are more attenuated from pole to pole when paralleling the equatorial plane. But, the central region of the inner core has a distinctorientation of anisotropy, and the outermost region is almost isotropic. Large-scale lateral heterogeneities occur both in latitude and longitude in the outer portions of the inner core; seismic velocities are higher, and seismic waves exhibit more attenuation in the eastern hemisphere than in the western hemisphere. There is also fine-scale (few kilometer) heterogeneity within the inner core. All of this complexity is unexpected, and has prompted mineral physics and geodynamic modeling of high-pressure iron phases and crystallization mechanisms.
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Inner core heterogeneity has been exploited to detect small differences in rotation of the inner core relative to the mantle. The differential rotation was detected by observations of systematic changes in the travel time of P-waves transmitted through the inner core over several decades (a strong argument for longterm operation of high-quality seismic observatories). The travel time changes are very small, involving tenths of a second difference on the same path traversed decades apart. Travel time changes have also been observed for reflections off the inner core surface, indicating temporal changes of the near surface. The source of the torques driving either a differential rotation or wobble of the solid inner core may be electromagnetic in origin and related to time variations in fluid flow or gravitational in origin and related to heterogeneities at the base of the mantle. Changes in local inner core surface conditions could arise due to lateral variations in outer core convection or inner core growth rates. Combined seismological, petrological, and geomagnetic studies are needed to shed light on the mechanism of growth of the inner core, which adds centimeters per century to its radius. This process of growth is critical to the thermal evolution of the core, the cooling history of the planet, and the geodynamo.


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