Web Extra     September 7, 2001

New directions in inner core research

When an earthquake occurs, elastic waves travel through Earth’s iron inner core faster in the north-south direction than in the east-west direction.  For more than 15 years, scientists have struggled to understand this strange property of P waves called seismic anisotropy. Two studies published this week in Nature examine iron’s high-temperature crystalline structure to shed new light on anisotropy, and, in turn, Earth’s inner core.

Gerd Steinle-Neumann and Lars Stixrude from the University of Michigan, Ronald Cohen from the Geophysical Laboratory of the Carnegie Institution of Washington and Oguz Gülseren from the National Institute of Standards and Technology and the University of Pennsylvania used supercomputers to model the high-pressure, hexagonal close-packed structure of iron at temperatures between 4000 and 7000 K. Steinle-Neumann and his colleagues found that the sides of the hexagonal prism grow with temperature, while the hexagonal bases shrink, changing the rigidity of the prisms. As the sides lengthen and bases shrink, the hexagon is looser and easier to disturb in the long direction, and more rigid and difficult to disturb in width. Using these newly calculated elastic constants, Steinle-Neumann and his co-workers created a general model of inner core structure in which the hexagonal bases preferentially align with Earth’s rotational axis. “The changes were unexpected from a material physics point of view, and with these new elastic constants, we can reconcile apparent difficulties we’ve had in the past,” Steinle-Neumann says.

The elastic constants provided the basis for microstructure modeling of many iron crystals in the inner core developed by Bruce Buffett at the University of British Columbia and Hans-Rudolph Wenk at the University of California at Berkeley. Buffett and Wenk propose a new forcing mechanism on the crystals: electromagnetism acting on the inner core from the generation of Earth’s magnetic field in the outer core. They generated a physical and numerical model, called a texture model, to describe the crystals’ deformation process, or flow under magnetic stress. Calculations of the average seismic wave speeds using the texture model, and the new elastic constants of Steinle-Neumann and others, produce an anisotropic difference that agrees with seismic observations. Buffett says the new modeling “may be a first step” in understanding Earth’s inner core dynamics and history.

Lisa M. Pinsker

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