The inner inner core of Earth

Abstract

The solid inner core (Fig. 1) is the most remote and enigmatic part of our planet, and, next to the crust, is the smallest "official" subdivision of Earth's interior. It was discovered in 1936 (1), and by 1972 it was established that it was solid, albeit with a very small rigidity (2–4). By 1993 it had been established that it was crystalline (5). The inner core is isolated from the rest of Earth by the low-viscosity fluid outer core, and it can rotate, nod, wobble, precess, oscillate, and even flip over, being only loosely constrained by the surrounding shells. Its existence, size, and properties constrain the temperature and mineralogy near the center of the Earth. Among its anomalous characteristics are low rigidity and viscosity (compared with other solids), bulk attenuation, extreme anisotropy, and superrotation (or deformation; refs. 5–8). From seismic velocities and cosmic abundances, we know that it is composed mainly of iron-nickel crystals, and the crystals must exhibit a large degree of common orientation. The inner core is predicted to have very high thermal and electrical conductivity, a nonspherical shape, and frequency-dependent properties; also, it may be partially molten. It may be essential for the existence of the magnetic field and for polarity reversals of this field (D. Gubbin, D. Alfe, G. Masters, D. Price, and M. Gillan, unpublished work). Freezing of the inner core and expulsion of impurities is likely responsible for powering the geodynamo. Yet, the inner core represents less than 1% of the volume of Earth, and only a few seismic waves ever reach it and return to the surface. The inner core is a small target for seismologists, and seismic waves are distorted by passing through the entire Earth before reaching it.

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