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Earth's Seismic Wave Path Based On Crystals In Core

Scientists have long known that the Earth's core, which generates the planet’s magnetic field, is primarily composed of iron. Its center portion -- the inner core -- is a solid sphere, which over the course of Earth's history has grown to its present size of 1,200 km.

Seismological observations have shown that elastic waves generated from earthquakes travel through the inner core faster along directions parallel to Earth's rotation axis than in the direction of the equator.

The cause of this anisotropy is not well understood, partly because the elastic properties of iron at the high pressure and temperature of Earth's center are not known.

In today's issue of Nature, 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 report their surprising results for the high-pressure structure of iron at very high temperatures (4000-7000 K).

The crystal structure of iron at high pressure is the hexagonal close packed structure; alternating layers of atoms are fitted as if packing layers of oranges in a highly efficient manner, giving a repeat unit in the shape of a hexagonal prism.

This work promises to advance our understanding about the origin of Earth's core, the generation of its magnetic field, and the dynamics for much of the planet’s interior.

While the extreme conditions of the inner core cannot readily be achieved in the laboratory for measuring seismic velocities, researchers can predict the properties of iron at core conditions by performing first-principles calculations on supercomputers. These calculations rely on the fundamental properties of physics and are free of experimental input.

In 1999, Walter Kohn received the Nobel Prize in Chemistry for the development of the theory on which these methods are based (density functional theory).

Caused by a change in the shape of the hexagonal prisms of the crystal structure, the scientists found to their own surprise that the elastic properties of iron at high temperature are quite different from those at low temperature. (The sides of the hexagonal prism grow with temperature while the hexagonal bases themselves shrink.)

This finding calls into question previous explanations for the seismological observations, which were based on the low-temperature properties.

The results do, however, support the hypothesis that the directional behavior in seismic wave propagation is an expression of alignment of crystals in the inner core.

The process of alignment is facilitated by stresses acting on the inner core for which various models exist. Based on an examination of such stress fields, Steinle-Neumann and co-workers developed a simple model of inner-core structure in which the hexagonal bases are preferentially aligned with Earth's rotation axis.

The strong temperature dependence of the average seismic wave velocity in iron and an almost perfect agreement of such properties with those of the inner core at a temperature of 5700 K also led the authors to infer that this is the temperature in the center of the Earth.

Refined models of the dynamics in the inner core based on the calculated high-temperature elasticity of iron should help to finally establish the cause of seismic anisotropy.

The first applications of these results are published in the same issue of Nature by Bruce Buffett (University of British Columbia) and Hans-Rudolf Wenk (University of California at Berkeley), making promising steps towards the identification of mechanisms involved in crystal alignment in the inner core.

Theoretical studies such as this are increasingly complementing experimental and observational work in the earth sciences, material physics and chemistry. They have successfully provided insight into the microscopic cause for many physical phenomena, predicted material properties, and expanded the range of conditions under which materials have been studied.

Gerd Steinle-Neumann is a graduate student at the Department of Geological Sciences at the University of Michigan in Ann Arbor, finishing up his PhD thesis. His advisor, Lars Stixrude, is a professor there since 1997. Both also hold positions as visiting investigators at the Geophysical Laboratory of the Carnegie Institution of Washington, where Ronald Cohen is a staff member.

During completion of this work, Ronald Cohen was also a visiting professor at the Seismological Laboratory and the Department of Material Sciences at the California Institute of Technology.

The collaboration with Oguz Gülseren started when he was a research associate at the Geophysical Laboratory; he moved on to the Center of Neutron Research at the National Institute of Standards and Technology and the Department of Material Sciences and Technology of the University of Pennsylvania during the completion of the research presented here.

Others involved in related investigations in high-pressure research are Russell Hemley, Carnegie Institution of Washington, Raymond Jeanloz, Univ. of California, Berkeley, Hans-Rudolf Wenk, Univ. of California, Berkeley.

Scientists studying geophysics of the inner core included Bruce Buffett, Univ. British Columbia, Xiadong Song, University of Illinois, Peter L. Olson, Johns Hopkins University and Shun-Ichiro Karato, Yale University.

[Contact: Gerd Steinle-Neumann, Lars Stixrude, Ronald Cohen]






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