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Structure, Composition and Dynamics of the Earth’s Core

Barbara Romanowicz
p. 45

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Source: La lettre, no. 33, May 2012

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1This international symposium gathered specialists from different disciplines within the earth sciences to review recent theoretical, experimental and observational findings which have furthered our understanding of the current physical and chemical characteristics of the Earth’s core and of its evolution on a geological timescale.

2Since the discovery of the inner core’s anisotropy in 1986, significant efforts in seismology have gone into the detailed spatial mapping of this inner core and of the underlying isotropic structure (presentation by Dr Jessica Irving, Cambridge University). A model recently proposed and presented by Dr Annie Souriau (Midi-Pyrénées Observatory, Toulouse) interprets these observations in terms of the inner core’s asymmetrical growth, associated with a progressive east to west translation of the iron crystallisation front.

3The use of anvil presses within particle accelerators (synchrotrons) recently allowed for the development of in situ techniques to investigate materials under high pressure and temperatures – up to the conditions of the core – based on X-ray diffraction. Dr Daniele Antonangeli (IPG/UPMC) discussed recent progress in the experimental determination of the elastic properties of iron and its alloys to deduce, on the basis of comparison with seismic data, which light elements are present in the core and the inner core and in what proportions – a subject that has been highly controversial for several decades now. According to Dr Antonangeli, among the light elements generally proposed (oxygen, silicon, sulphur, carbon and hydrogen), silicon could be the dominant element in the inner core. Furthermore, various mechanisms were put forward to reconcile the elastic shear wave speeds recorded in the inner core with those predicted by materials physicists, which are significantly higher (10%). Based on theoretical calculations (ab-initio), Dr Vocadlo offered a simple explanation based on the previously neglected yet important effects of nickel on the elastic properties of the iron-nickel alloy in the inner core, in addition to the light elements already mentioned.

4Through the comparison of the abundance of chemical elements observed in the Earth’s mantle and in the solar system, the kinetics of oxido-reduction reactions, and through the quantification of the isotopes of radioactive elements, geochemistry/cosmochemistry also contributes to the constraints on the mode of formation, the age of the key events of the Earth’s history, such as the giant impact at the origin of the moon’s formation (presentation by Dr Bernard Bourdon), and the light element composition of the core (presentation by Dr Michael Walter).

5Finally, the different geophysical and geochemical constraints serve as a basis for the elaboration of models describing the evolution over time of dynamic processes, such as the inversions of the magnetic field (presentation by Dr Julien Aubert) and the growth of the inner core (presentation by Dr Philippe Cardin). The former suggests a data assimilation approach to build digital models of the Earth’s dynamo, while the latter proposes a simple model of convection/translation inside the inner core to explain the seismic anisotropy observed and its hemispherical asymmetry.

6The eight presentations were followed by a general discussion organized around a panel and a poster session, where PhD students from several Geophysics doctoral schools (IPGP, ENS, Paris-Sud Orsay) presented a summary of current research on a theme of their choice relating to the Earth’s core. This poster session illustrated a possible way of involving students from the region’s doctoral schools in teaching at the Collège de France.

• Lectures and audios available online at www.college-de-france.fr

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Référence électronique

Barbara Romanowicz, « Structure, Composition and Dynamics of the Earth’s Core », La lettre du Collège de France [En ligne], 7 | 2015, mis en ligne le 02 novembre 2015, consulté le 23 mai 2017. URL : http://lettre-cdf.revues.org/2692

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Barbara Romanowicz

Physics of the Earth’s Interior

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