The Enigmatic Inner Core

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Science  21 May 2010:
Vol. 328, Issue 5981, pp. 982-983
DOI: 10.1126/science.1190506

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Over the past billion years or so, Earth's liquid iron core has cooled and solidified, resulting in the growth of a solid inner core (1). Seismic data on the structure of this inner core reveal a bewildering degree of complexity (2, 3). Most surprising is the observation that the seismic properties differ between the eastern and the western half of the inner core (see the figure) (4). How could distinct hemispheres develop as the inner core has grown? On page 1014 of this issue, Monnereau et al. (5) propose an intriguing answer: They suggest that as the inner core has grown, material has moved laterally within it. When this lateral motion is sufficiently fast, melting and solidification should occur in opposite hemispheres. Newly formed iron crystals in one hemisphere would grow with age during transit across the inner core. The hemispherical differences in seismic properties emerge from differences in average crystal sizes. Further insight into the properties of the inner core is provided by Deuss et al. on page 1018 (6), who develop a new method for estimating hemispherical structure.