Structure and dynamics of Earthʹs inner core

The seismological exploration of the Earthʹs inner core has revealed unexpected and puzzling structural complexities. Its elastic anisotropy is now well established, and has been shown to vary spatially. There is a well defined hemispherical dichotomy in anisotropy and attenuation, and significant variations in the radial direction, with weaker anisotropy in the uppermost regions, and possibly different properties in an “innermost inner core” of radius 300–600 km. Perhaps even more puzzling is the observation of a stably stratified layer at the base of the outer core, which has recently been proposed to result from melting of inner core material. If confirmed, the presence of this layer has major implications for our understanding of convection in the outer core. Inner core differential rotation with respect to the mantle remains controversial. We will review here recent progress in understanding the dynamics of the inner core. A number of different models have been proposed for the development of the inner coreʹs structure and formation of the stable layer at the base of the outer core, and particular care will be taken to clarify under which conditions the proposed mechanisms can be active.