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Is the core pure iron no more?

Is the core pure iron no more?

Our planet has a heart of iron. In the 50 years since the earth'sinner core was discovered, scientists have concluded that while the liquid outer core is made of an iron alloy, the solid inner core is pure, crystalline iron.

Now, in the Jan. 22 NATURE, two geophysicists argue that theinner core is not as pristine as once thought. Andrew Jephcoat at the Carnegie Institution of Washington (D.C.) and Peter Olson at the Johns Hopkins University in Baltimore conclude that some element lighter than iron, such as sulfur or oxygen, is also contained in the inner core. They arrived at this conclusion by comparing the inner core density, as calculated by seismologic studies, with the densities estimated from laboratory measurements of pressurized iron and iron sulfide samples.

According to Raymond Jeanloz at the University of Californiaat Berkeley, who comments on the paper in NATURE's News and Views section, if Jephcoat and Olson are correct, then further work may enable researchers to determine the temperature at which the inner and outer core compositions coexist. And since the inner core is small, he adds, this is akin to finding the temperature at the center of the earth.

Jephcoat and Olson's conclusion may also help scientistsunderstand the energy source that drives fluid motions in the outer core (which generate the planet's magnetic field) as well as the convection of the material in the mantle. A major theory holds that outer core motions are driven by compositional differences between the inner and outer cores; dense, iron-rich crystals solidify from the less dense, alloy liquid and sink toward the inner core, stirring up the outer core fluids in the process.

Jephcoat and Olson write that compositional differencesmay still contribute to outer core fluid motions, although the compositional differences between the inner core and the outer core are not as great as scientists once thought. But because the compositional mechanism is most efficient at low core temperatures, they favor another theory in which the radioactive decay heats the core and the mantle, driving convection in both regions.
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Title Annotation:earth's core
Publication:Science News
Date:Feb 14, 1987
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