Other
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.p53b..02l&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #P53B-02
Other
5400 Planetology: Solid Surface Planets
Scientific paper
Over a decade ago, we reported on viscosity estimates for the Earth's fluid outer core (Lumb & Aldridge, J. Geomag. Geoelectr., 43, 93-110, 1991). At that time, estimates for this fundamental deep-Earth parameter ranged over 14 orders of magnitude. Opposite this range we identified: the prevailing wisdom of dismissing higher estimates as upper bounds and taking lower estimates as reliable; self-consistency amongst estimates based on theoretical and laboratory studies of liquid metals (LM), but ranges in estimates inferred from real-Earth (RE) observations; anomalous bulk viscosities, which owe their existence to multiphase fluids, as a plausible explanation for high-value seismically derived estimates; and LM estimates as representative of the molecular viscosity of the outer core, with RE estimates as eddy or effective viscosity estimates. Although each of these findings has been amplified by others, recent results encourage us to revisit estimates for this important physical parameter. In particular, theoretical (e.g., First Principles Molecular Dynamics simulations) and laboratory (e.g., in situ X ray radiography of diamond-anvil experiments) results for liquid-metal alloys (LMAs) exhibit striking self-consistency for increasingly realistic core compositions. Even when extrapolated to realistic core conditions, these LMA results are still orders of magnitude less than results derived from RE observations. Viscosity estimates from the translational motion and differential rotation of the Earth's inner core now complement those estimates derived from other RE observables. Although these real-Earth estimates based on dissipative processes still range over several orders of magnitude, there is some indication of a systematic variation of viscosity with the timescale of the dissipative process. Confirmation of this systematic variation has significant implications. For example, it may form the basis for positing the Earth's fluid outer core as a thixotropic fluid in the broadest sense - i.e., a fluid whose viscosity is modified when subjected to a disturbance. We report here on these recent results, and this systematic variation, in the context of core-mantle coupling and geodynamo modeling.
Aldridge Keith D.
Lumb Ian
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