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Sneaking a peek at earth's hardened heart.

While Earth's iron core lies only 2,900 kilometers below our feet, it has proved less accessible than the outer reaches of the solar system. This week, two research groups report results that offer new insight into Earth's hidden heart.

Geophysicist Bruce A. Buffett and his colleagues at the University of Cambridge in England have for the first time succeeded in capturing some of the core's history within a mathematical formula. The researchers investigated the growth of Earth's inner core -- a region squeezed by such intense pressures that it remains a solid despite the tremendous heat deep inside the planet.

Surrounding this hard center lies the outer core, a swirling pool of molten iron and other "lighter" elements. Geoscientists believe that liquid iron from the outer core gradually solidifies onto the inner core, steadily building up that solid center at a current rate of about 0.4 millimeters per year.

The Cambridge group's analysis addresses a central question about the Earth: What processes stir the liquid outer core? The movement of these fluids is important because they apparently generate Earth's magnetic field -- one of the factors that makes this planet hospitable to life.

Researchers have long thought that heat differences drive the fluid motion within the outer core, much like the roiling convection of soup on a stove. In this theory, heat escaping up into the mantle would cool off the upper regions of the outer core, causing fluid to sink. At the base of the outer core, molecules of liquid iron would release energy as they solidified, heating the lowermost fluid until it rises.

Some gephysicists have advocated another theory. Instead of focusing on heat, they suggest that the freezing of iron on the inner core would leave behind lighter elements in the fluid, which would rise to the top of the outer core, causing convection.

The work by Buffett's group suggests that the separation of light elements and the cooling at the top of the core play roughly equal roles in stirring the outer core. Heat released by solidifying molecules represents a minor factor, they report in the March 26 NATURE.

Buffett and his colleagues estimate that the inner core began growing sometime between 3.6 and 1 billion years ago, depending on the rate at which heat escapes from the core into the mantle. Such calculations suggest that Earth's magnetic field may have developed before the birth of the inner core, because rocks dating back 3.5 billion years contain a record of the magnetic field.

David E. Loper at Florida State University in Tallahassee cautions that the analysis by Buffett's group does not resolve major uncertainties. "Their numbers aren't that reliable. We don't know enough about the thermal history of the mantle and core to tie these numbers down."

While the core's history remains fuzzy, its structure is gradually coming into focus through the use of seismology. Kenneth C. Creager at the University of Washington in Seattle compared seismic waves that passed through the edge of the solid inner core with almost identical ones that passed just outside the inner core. Rays passing through the inner core traveled 3.5 percent faster when they were moving parallel to Earth's spin axis, Creager reports in the same issue of NATURE.

Seismologists had detected this so-called anisotropy before, but previous work had not detected such a marked difference in speeds. To explain his findings, Creager suggests that iron crystals within the inner core have aligned themselves in an orderly fashion that affects the speed of seismic waves, depending on their direction. At present, though, geoscientists do not know why the crystals would position themselves in an orderly pattern rather than a random one.
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Author:Monastersky, Richard
Publication:Science News
Date:Mar 28, 1992
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