Elasticity and rheology of iron above 220 GPa and the nature of the Earth's inner core

Recent numerical-modelling and seismological results have raised new questi ons about the dynamics(1,2) and magnetism(3,4) of the Earth's core. Knowled ge of the elasticity and texture of iron(5,6) at core pressures is crucial for understanding the seismological observations, such as the low attenuati on of seismic waves, the low shear-wave velocity(7,8) and the anisotropy of compressional-wave velocity(9-11). The density and bulk modulus of hexagon al-close-packed iron have been previously measured to core pressures by sta tic(12) and dynamic(13,14) methods. Here we study, using radial X-ray diffr action(15) and ultrasonic techniques(16), the shear modulus, single-crystal elasticity tensor, aggregate compressional- and shear-wave velocities, and orientation dependence of these velocities in iron. The inner core shear-w ave velocity is lower than the aggregate shear-wave velocity of iron, sugge sting the presence of low-velocity components or anelastic effects in the c ore. Observation of a strong lattice strain anisotropy in iron samples indi cates a large (similar to 24%) compressional-wave anisotropy under the isos tress assumption, and therefore a perfect alignment of crystals(6) would no t be needed to explain the seismic observations. Alternatively the strain a nisotropy may indicate stress variation due to preferred slip systems.