Abstract
Seismological studies1,3 suggest that there is 3-4% anisotropy of P-waves in the Earth's inner core. Hexagonal closed-packed solid iron has been proposed to be the major constituent of the inner core4, but a lack of knowledge about the elastic properties of this phase at inner-core pressures (330 to 360 GPa) and temperatures (4,000 to 8,000 K) prevents a conclusive interpretation of the seismic results in terms of the inner-core composition. The elastic properties of hexagonal closed-packed iron have been computed from first principles using a theoretical method4 and Mao et al.5 have recently measured them by using X-ray diffraction and ultrasound techniques at pressures of up to 211 GPa. Despite these exciting technological achievements, we believe that the new results may be misleading.
Main
Although the theoretical and experimental results both indicate similar aggregate P-wave and S-wave velocities as a function of pressure5, there is an inconsistency in the elastic constants that Mao et al.5 have attempted to resolve by multiplying the P-wave velocities at 16 and 211 GPa by 1.08. However, this treatment results in a 15-40% change in elastic parameters at 211 GPa and is accompanied by a 7-30% decrease in the S-wave velocity (Fig. 1). The authors assume that the values at 39 GPa are correct, but even at that pressure some of the elastic parameters have changed by up to 15% (Fig. 1). Furthermore, in their correction, Mao et al.5 obtain a value for the elastic constant C 33of 491 GPa, which is 63% less than the value of 802 GPa obtained at 39 GPa pressure4. Such a low value for C 33yields a very low Poisson's ratio (0.04) along the symmetry axes. Mao et al. 's5 value for C 44is 38% higher than that obtained at 211 GPa pressure4.
Our analysis suggests that the uncertainty in the individual elastic constants is much greater than in parameters based on a combination of them, such as the P-wave velocity. Given the large uncertainties and the physical implications of the elastic parameters, the results of Mao et al.5 must be interpreted with caution.
References
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Mao, H.et al. Nature 396, 741–743 (1998); correction, ibid 399, 280 (1999).
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Singh, S., Montagner, JP. Anisotropy of iron in the Earth's inner core. Nature 400, 629 (1999). https://doi.org/10.1038/23171
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DOI: https://doi.org/10.1038/23171
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