Under Pressure

December 23, 1998

Scientists at the Carnegie Institution of Washington have published in the December 24 issue of Nature results of a high-pressure study of the material that exists at the Earth's interior. By using X-ray diffraction techniques with diamond anvil cells, Ho-kwang Mao, Russell Hemley and colleagues1 at the Geophysical Laboratory have, for the first time, determined the elastic properties of iron under ultrahigh pressure. Their work suggests that the inner core propagates sound in the same manner as highly compressed iron near its melting temperature.

The Earth's core is found between 2890 and 6370 km depth, which corresponds to pressures between 1.3 and 3.6 million times atmospheric pressures. The outer region is liquid, while the inner core is solid. The inner core is where scientists have observed some peculiar phenomena. Seismologists have found that sound waves traveling through the inner core in an east-west direction are slower than those traveling in a north-south direction. This difference, called seismic anisotropy, could arise from texture likened to the grain in wood, and the ease with which wood will split in one direction. Among scientists, there has been much speculation as to what the texture of the core could be like to produce this difference in seismic waves.

Previously, scientists could not measure experimentally the elasticity of iron and other materials at the extreme pressures that exist deep within the Earth. Recently, however, the Carnegie group developed a novel X-ray diffraction method that allows the determination of elasticity of iron, from which the sound velocity (what is measured by seismologists) can be determined. The researchers found that the elasticity of hexagonal-close-packed iron, which appears to be the form of iron under these extreme pressures, can account for the slow seismic wave velocity at the inner core. They also suggest that the speed of the seismic waves points to an inner core that is close to melting. The authors also note that an alternative scenario is possible: that the presence of additional components with low shear-wave velocities and densities similar to iron could also be present in the inner core.

High-pressure research has made tremendous contributions to the earth sciences in the past few years, as instrumentation capabilities continue to improve. Many of these techniques involve the use of diamond anvil cells and intense synchrotron radiation sources of X-rays. By studying the crystal structure, texture, and elasticity of iron at extremely high pressures, scientists can progress toward complete comprehension of the Earth's interior, and, consequently, apply this understanding of phase relations to the study of other planets.

The National Science Foundation supported this research, in part through the Center for High- Pressure Research, an NSF Science and Technology Center.

1Guoyin Shen, University of Chicago, Baosheng Li, SUNY Stony Brook, and Anil K. Singh, National Aerospace Laboratories, India.
The Geophysical Laboratory, established in Washington, D.C. in 1907 and led today by Wesley T. Huntress, is one of five research arms of the Carnegie Institution of Washington. Since its founding, high pressure and high temperature experimentation has been a research cornerstone. The Carnegie and Stony Brook authors of this study are members of the NSF Center for High-Pressure Research, based at the Geophysical Laboratory. The Carnegie Institution is led by Maxine F. Singer.

Carnegie Institution for Science

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