Direct observations of the viscosity of Earth's outer core and extrapolation of measurements of the viscosity of liquid iron

D.E. Smylie^a, V.V. Brazhkin^b, A. Palmer^c
^aDepartment of Earth and Space Science and Engineering, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3, Canada
^bInstitute for High Pressure Physics, Russian Academy of Sciences, Kaluzhskoe shosse 14, Troitsk, Moscow, 108840, Russian Federation
^cSander Geophysics Ltd., 260 Hunt Club Road, Ottawa, Ontario, K2P 1K2, Canada

Estimates vary widely as to the viscosity of Earth’s outer fluid core. Directly observed viscosity is usually orders of magnitude higher than the values extrapolated from high-pressure high-temperature laboratory experiments, which are close to those for liquid iron at atmospheric pressure. It turned out that this discrepancy can be removed by extrapolating via the widely known Arrhenius activation model modified by lifting the commonly used assumption of pressure-independent activation volume (which is possible due to the discovery that at high pressures the activation volume increases strongly with pressure, resulting in 10² Pa s at the top of the fluid core, and in 10¹¹ Pa s at its bottom). There are of course many uncertainties affecting this extrapolation process. This paper reviews two viscosity determination methods, one for the top and the other for the bottom of the outer core, the former of which relies on the decay of free core nutations and yields 2371 ± 1530 Pa s, while the other relies on the reduction in the rotational splitting of the two equatorial translational modes of the solid inner core oscillations and yields an average of 1.247 ± 0.035 × 10¹¹ Pa s. Encouraged by the good performance of the Arrhenius extrapolation, a differential form of the Arrhenius activation model is used to interpolate along the melting temperature curve and to find the viscosity profile across the entire outer core. The viscosity variation is found to be nearly log-linear between the measured boundary values.

PACS: 66.20.−d, 91.35.−x, 93.85.−q (all)
DOI: 10.3367/UFNe.0179.200901d.0091
URL: https://ufn.ru/en/articles/2009/1/d/
Web of Science

000266708800004
Scopus

2-s2.0-67849111476
ADS NASA

2009PhyU...52...79S
Citation: Smylie D E, Brazhkin V V, Palmer A "Direct observations of the viscosity of Earth's outer core and extrapolation of measurements of the viscosity of liquid iron" Phys. Usp. 52 79–92 (2009)

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Оригинал: Смайли Д Е, Бражкин В В, Палмер А «Прямые наблюдения вязкости внешнего ядра Земли и экстраполяция измерений вязкости жидкого железа» УФН 179 91–105 (2009); DOI: 10.3367/UFNr.0179.200901d.0091

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