Earth's deepest secrets

September 05, 2001

ANN ARBOR---In work that promises to advance understanding about the origin and dynamics of Earth's iron-rich inner core and the generation of the planet's magnetic field, a team that includes University of Michigan researchers has found that the elastic properties of iron are quite different at extremely high temperatures than at low temperatures.

The surprising finding, reported in the September 6 issue of the journal Nature, calls into question previous interpretations of seismic images that were based on iron's low-temperature properties. The correct interpretation of such images---created by mapping the behavior of waves of energy that shake the ground during earthquakes---is a key to understanding the structure of Earth's interior.

Scientists have long known that Earth's core, which is responsible for generating 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 (about 745 miles). But researchers have been puzzled by the observation, based on seismological measurements, that elastic waves generated by earthquakes travel through the inner core faster along directions parallel to Earth's polar axis than in other directions. The cause of this difference has not been well understood, partly because the elastic properties of iron at the high pressure and temperature of Earth's center are not known (it is impossible to take direct measurements at the core, and core conditions are difficult to duplicate in the laboratory).

Researchers can, however, predict the properties of iron at core conditions by performing simulations on supercomputers. Rather than relying on direct measurements or experimental data, these simulations are based on the fundamental properties of physics. In the work reported in Nature, graduate student Gerd Steinle-Neumann and Lars Stixrude, professor of geological sciences, from the U-M, 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 supercomputer simulations to study changes in the crystal structure of iron at high pressure and very high temperatures of 4,000-7,000 K (6,740-12,140 F). Changes in the basic hexagonal prism shape of the crystals directly influence elastic properties, Steinle-Neumann explains. To the researchers' surprise, the elastic properties of iron were quite different at high temperatures than at low temperatures, an observation that should lead to new interpretations of seismic images.

The results support the hypothesis that the directional behavior in seismic wave propagation reflects the alignment of crystals in the inner core. Stresses acting on the inner core influence the alignment process, and various models have been proposed to explain how that occurs. Steinle-Neumann and co-workers surveyed a number of those models and developed their own, simple model of inner-core structure, in which the hexagonal bases of the crystals tend to align with Earth's polar 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 5,700 K (9,800 F) also led the authors to infer that this is the temperature in the center of the Earth.

The researchers hope that other scientists will use this new understanding of the high-temperature elasticity of iron to refine models of the dynamics in Earth's inner core. These refined models should help to finally explain why seismic waves travel faster in particular directions.

"We are characterizing the material properties, which ultimately will help us understand the dynamic processes" that underlie the differences in seismic wave velocity, says Steinle-Neumann. The first applications of these results are published in the same issue of Nature by Bruce Buffett of the University of British Columbia and Hans-Rudolf Wenk of University of California at Berkeley.

Theoretical studies such as those of Steinle-Neumann and his co-workers are increasingly complementing experimental and observational work in the Earth sciences, material physics, and chemistry. They have successfully provided insight into the microscopic causes of many physical phenomena, predicted material properties, and expanded the range of conditions under which materials have been studied.
This work was supported by the National Science Foundation and the U.S. Department of Energy.

The University of Michigan
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University of Michigan

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