The Earth's inner core consists of upper and lower regions with different material properties, and is not a uniform crystal of iron, as scientists had thought, according to seismologists at Columbia University's Lamont-Doherty Earth Observatory and the California Institute of Technology.
The findings are likely to affect the current model of how the Earth and its magnetic field came into being.
The researchers, Xiaodong Song, the Storke-Doherty Lecturer at Lamont, Columbia's earth sciences campus in Palisades, N.Y., and Don V. Helmberger, a seismologist and professor at California Institute of Technology in Pasadena, Calif., used earthquake data to probe the inner core. Their work was supported by the National Science Foundation and appears in the Oct. 30 issue of the journal Science.
The inner core of the Earth is a 1,500-mile-wide sphere comprised mainly of solid iron that rotates in an outer core of molten iron. While scientists already know of a boundary between the inner and outer cores, the research conducted by Dr. Song and Dr. Helmberger demonstrates layers within the inner core itself, which is located some 3,000 miles below the Earth's surface. The scientists used seismic data from 11 historic earthquakes to infer that the inner core has two distinct parts: a lower area surrounded by a thin, uneven upper layer with different material properties. In that respect, the inner core is much like the Earth's upper layer, the mantle, which has upper and lower regions with different mineralogy.
The researchers obtained measurements made in Germany of seismic waves traveling west to east -- from earthquakes in Fiji and other South Pacific islands. Waves traveling south to north -- from earthquakes in the region off the southern coast of South America -- were measured in Alaska and Canada.
Waves from the South Pacific earthquakes were detected first in North America and later in Germany, indicating that the waves moved faster from south to north than from east to west. The waves moved faster, the researchers said, because iron crystals in the Earth's inner core are arranged in a way that allows waves to move faster in one direction that the other.
The team's measurements showed that waves from certain earthquakes reached some northern seismographs before others. Using computers to simulate waves propagating through the Earth, the pair narrowed the source of these differences to the inner core and discovered a point of transition 120 miles below its surface. Above that point, the inner core was isotropic, meaning waves could travel equally fast in any direction. Below it, anisotropy hindered waves moving in some directions and promoted them in others. The boundary between the upper and lower inner core regions appears to vary in depth at different places beneath the Earth's surface, the researchers said.
The path taken by seismic waves through the inner core is analogous to that taken by light waves passing across an air-water interface, and demonstrates the existence of an inner core boundary, Dr. Song said.
"It is like placing a straw in a glass of water," he said. "When looking at it from the side of the glass, the straw appears bent. The light is refracted, bent a little as it passes through the water to the air.
"In the same manner, the seismic wave is refracted as it passes through the anisotropic layer of the core and back to the isotropic layer that surrounds it."
Drs. Song and Helmberger hypothesize that iron crystals in the upper part of the inner core are randomly oriented, while crystals in the lower portion are aligned in a north-south pattern, meaning that waves going from south to north will move faster than those traveling west to east, the phenomenon the team observed. The cause of such layering is unclear, but the researchers proposed that pressure, thermal and magnetic forces in action at the center of the earth cause the upper layer to change shape over time.
Dr. Song's recent work has been animated by interest in the inner core. Two years ago, Dr. Song, working with Paul Richards, Mellon Professor of Natural Sciences at Columbia, found evidence that the inner core is spinning faster than the Earth itself. The rotation of the inner core is caused by the electromagnetic torque exerted on the conducting inner core by the magnetic field generated in the outer core. By understanding the structure of the inner core, Dr. Song said, scientists could gain a better understanding of the role of the inner core on a number of earth processes, such as the generation of the Earth's magnetic field, interactions between the different geologic layers and the differentiation and evolution of Earth's interior.
Dr. Song cited the current lack of critical data as a major limiting factor of future studies of the inner core. More stations in locations at high latitudes, such as Alaska or Antarctica, are needed to record waves in north-south directions, which would facilitate future research examining the structure and processes of the inner core, he said.