How hot is the heart of the earth?

How hot is the heart of the earth?

If the earth were all one temperature,the surface of the planet would be an uninteresting place indeed. The temperature differences and heat flow in the earth's interior power volcanoes and earthquakes as well as the formation of mountains and oceans. Measuring the temperature profile of the earth has, therefore, been an important goal in the geosciences, but until recently scientists' ability to determine the temperature of the planet, especially its deepest regions, has been lukewarm at best.

Now things are heating up. Technologicaladvances are enabling scientists to study materials under the extreme pressures and temperatures found in the inner earth. With these advances, says Quentin Williams, a graduate student at the University of California at Berkeley, "we've been able, for the first time, to [find] the melting temperature of iron-- the dominant material in the earth's core and probably in planetary cores in general --to pressures that actually exist within planetary interiors.'

In the April 10 SCIENCE, Williams andmineral physicist Raymond Jeanoloz at Berkeley, together with their colleagues at the California Institute of Technology in Pasadena, Calif., and the University of Illinois at Urbana-Champaign, report the results of the highest-pressure melting experiments ever performed on iron. From the observed melting temperatures of iron, the researchers say they have obtained the first experimentally determined upper limit on the temperature at the center of the earth.

Williams's group conducted two kindsof experiments. The Berkeley researchers studied the melting of iron in a laser-heated diamond cell to pressures of up to 100 billion pascals (GPa). In previous work using this technique, the maximum pressure was only 20 GPa, corresponding to a depth of 600 kilometers in the mantle. Williams says one of his group's main contributions has been to develop a system that can accurately measure the temperature at high pressures from the radiation spectrum emitted by the iron sample.

At Caltech, Bob Svendsen and ThomasJ. Ahrens subjected iron samples to short bursts of even higher pressures by firing plastic and tantalum bullets at them. They achieved pressures of 250 GPa, which is slightly greater than previous studies. This time, however, the researchers measured the temperature of the iron directly, without having to make assumptions about the iron's heat capacity and other thermodynamic parameters.

After 500 experiments, they determinedthat the melting point of iron at 136 GPa (comparable to the pressure at the core-mantle boundary) is 4,800 200 kelvins. They also determined that at 330 GPa (similar to the pressure at the boundary between the solid inner core and the liquid outer core), iron melts at 7,600 500 K. "These [values] are somewhat higher than previous estimates of the melting temperature of iron,' which were based in part on extrapolations from low-pressure data, says Williams.

The researchers estimate that the presenceof other elements, such as sulfur, would lower the melting points in the core by about 1,000 K. This means that the temperature at the top of the molten core must be greater than about 3,800 K. This value is 1,000 K higher than what scientists have calculated to be the temperature at the base of the mantle, says Williams. He adds that the temperature contrast between the outer core and lower mantle suggests that there is at least one nonconvecting layer in the mantle that is keeping heat from escaping too rapidly from the core.

As for the temperature at the center ofthe planet, the researchers' best estimate is that the solid inner core can be no warmer than 6,900 1,000 K.