Scrambled Earth; researchers look deep to learn how the planet cools its heart - includes related article about use of diamonds in high-pressure physics

Science News, April 9, 1994 by Richard Monastersky

In the beginning, there was heaven and a sizzling chunk of rock called Earth. Asteroids bombarded the surface of the infant planet, while radioactive elements seethed below, building up so much heat that most of the globe eventually melted. From a distance, the world would have looked like a giant drop of liquid rock circling the sun.

Since that time, Earth has slowly cooled, releasing much of its pent-up original heat. Even today, the quenching continues beneath our feet. The escaping energy pushes continents around the planet's surface through a process called plate tectonics. It causes the ground to quake and volcanoes to blow, killing thousands of people every year.

Yet for all its planet-wrenching consequences, this global cooling remains a mystery, For more than 2 decades, researchers have tried, with little success, to decipher exactly how Earth lets off steam.

The effort is beginning to pay off, however. Recent discoveries from several fields of research are now converging, allowing investigators to zero in on key issues. Tools as disparate as computer models and diamond-studded vises are bringing the planet's hidden interior into much sharper focus than ever before.

When reduced to its simplest form, the cooling question hinges on the mechanics of mixing. Like a pot of steaming soup, Earth loses heat through the stirring action of convective currents that draw hot material toward the surface, where it cools and then sinks. Much of this convection takes place within the mantle - the great rocky layer that surrounds the core and makes up 83 percent of the planet. Although the mantle is solid, the pressures are so intense that the deep stone actually flows, albeit at speeds of only a few centimeters per year.

Geophysicists can agree on that general picture, but arguments break out when they discuss how deep the mixing goes. Since the 1970s, scientists have debated two competing theories of convection. One camp, call them the lumpers, believes that convection currents stir the entire mantle, mixing both the upper and lower parts of this layer. Another group, call them the splitters, argues that the upper and lower mantle remain separate, each convecting on its own like the stacked pots of a double boiler. In this case, the upper mantle would act as a thermal blanket, insulating the lower mantle and slowing the escape of heat from the core.

The lumpers and splitters may both have it wrong, however. The newest findings suggest that Earth does not follow either of these simple patterns but might combine elements of both.

The current debate over mantle mixing extends an intellectual revolution that started nearly 30 years ago, when the theory of plate tectonics swept the geosciences. This powerful concept revealed that Earth's outer shell -- the lithosphere -- is broken into separate blocks that migrate around the globe like bumper cars in extra slow motion. Where two plates crash together, they build giant mountain ranges such as the Himalayas. When one plate slips beneath another, it creates a deep ocean chasm like the Mariana Trench. If two plates slip-slide past each other, they form quake-making faults like the San Andreas.

The theory of plate tectonics succeeded because it provided the intellectual framework to explain the planet's surface, the part that geologists can feel directly under their boot heels. But the theory only goes skin deep; it addresses just the top 100 kilometers of a planet that spans 6,370 km from surface to center. Even under the penetrating light of plate tectonics, the inner Earth has remained a terra incognita.

Scientists are therefore striving for deeper knowledge. "The new revolution is to project the global view of plate tectonics downward into the third dimension," says Raymond Jeanloz of the University of California, Berkeley

Because researchers cannot reach into the lower mantle to measure its properties, experimental geophysicists such as Jeanloz turn the problem around, bringing the inner Earth into the laboratory. Aiding them in this quest is the diamond anvil cell (see sidebar on p. 237), a bricksize instrument that allows researchers to recreate the hellish pressures and temperatures of the planet's center.

To address the convection debate, Jeanloz's group put the squeeze on a rock called periditite, which comes from the upper mantle. Although these rocks normally lie under Earth's surface crust, geologists find periditite exposed in places where interplate collisions have thrust part of the upper mantle above ground.

Jeanloz would like to work with actual material from the lower mantle, but no one knows what exists there. The rocks could contain the same ingredients as those of the upper mantle, or they could have a different composition.

As a test, Jeanloz used a diamond anvil to transport upper mantle periditite into a laboratory version of the lower mantle. The rationale vaguely resembles the Cinderella story. In that tale, if the prince can fit a foot into a glass slipper, he has found his princess. In Jeanloz's case, if the squeezed periditite fits the expected qualities of lower mantle rock, then he has found the long-sought stone that might fill this hidden layer of Earth.