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The current idea is that the Earth's outer core consists mainly of liquid iron, and that the convection of this metallic liquid is responsible for the Earth's magnetic field. However, a full understanding of the dynamics is hampered by uncertainty regarding the viscosity of the outer core, with viscosity estimates by various researchers ranging over 12 orders of magnitude. ... ... Wijs et al (7 authors at 3 installations, UK AT) present dynamical first principles simulations of liquid iron which indicate that the viscosity of iron at Earth core temperatures and pressures is at the low range of previous estimates -- roughly 10 times that of typical liquid metals at ambient pressure. The authors suggest this estimate supports the approximation commonly made in magnetohydrodynamic models that the outer core is an inviscid fluid undergoing small-scale circulation and turbulent convection, rather than large-scale global circulation. QY: Michael J. Gillan pha71@keele.ac.uk Related Background: EXPERIMENTAL APPROACHES TO THE ANALYSIS OF EARTH'S CORE Computer models of the Earth's core, combined with seismic information, have suggested that the iron crystals in the inner core are aligned, that the inner core has an intrinsic rotation, and that the rotation velocity is slightly faster than that of the rest of the planet. Seismologists have also suggested, on the basis of seismic studies, that the core is not isotropic. Gallium is a metallic element similar to mercury, with a low melting point (29.78 degrees Centigrade) and a high boiling point (2403 degrees Centigrade). In a review of experimental work in geophysics, A. Frank (Univ. of Rochester, US) emphasizes the advantages of experimental methods over computer simulations. Recent experiments involve using molten gallium as a model for the Earth's core, with measurements of the effects of local conditions such as the flow of heat, magnetic fields, and rotation on the grain of the liquid metal. Among other observat- ions, the Olson group (Johns Hopkins Univ., US) has apparently already provided evidence that temperature changes in Earth's core did not create the crystalline arrangements, and that it is rotation that may be the cause of the apparent core asymmetries. QY: Adam Frank, Univ. of Rochester, Dept. of Physics 716-275-4356 (Earth February 1988) (Science-Week 2 Jan 98) Related Background: ROTATION DIFFERENCES OF EARTH'S INNER CORE AND MANTLE Seismic wave propagations are the propagated shock waves produced by earthquakes, and quantitative analysis of these waves can tell us much about the structure of the Earth. Seismic studies indicate the interior of the Earth consists of three parts: a metallic core, a dense rocky mantle, and a thin low-density crust. The central part of the core is solid, but the outer part of the core is evidently liquid. The mantle, the layer of dense rock and metal oxides between the molten part of the core and the surface, has plastic properties (i.e., it is a solid capable of flow under pressure). Apparently, the Earth's magnetic field is a direct result of its rapid rotation and its molten core, and the theoretical account of this is called the "dynamo effect". The essential idea is that the liquid metallic core is stirred by convection, the rotation of the Earth couples this motion into a circulation that generates electric currents, and the electric currents in turn generate a magnetic field according to classical electromagnetic theory. K. Creager (Univ. Washington Seattle, US) reports a model that uses observations of particular seismic wave propagations and proposes that the inner core of the Earth is rotating 0.2 degrees to 0.3 degrees per year faster than the mantle. The author suggests this low difference raises the upper limit for inner core viscosity and thus constrains parameters for future dynamo models. QY: Kenneth C. Creager kcc@geophys.wash.edu |