Science: Journey to The Earth's Core

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Even though the center of the earth is closer to New York City than New York is to Honolulu, it is as inaccessible to scientists as the stars. Until recently, the earth's core, hidden under thousands of miles of rock, was a mystery. Now all that is changing. In the past two years, thanks to a technological revolution in methods of observation, scientists have begun to paint a theoretical portrait of the planet's interior in startling detail. Says Harvard University Geophysicist Adam Dziewonski: "For the first time we can actually see the inside of the machine."

What they "see" is astounding. Far from being just a featureless sphere of molten iron, the core has a surface that is apparently studded with mountains and riddled with depressions that may be filled with lower-density fluid that forms the equivalent of oceans. There may even be a bizarre kind of rain: showers of iron particles that sprinkle down on the core. And all of this takes place in a region whose temperature is perhaps as hot as the surface of the sun.

Describing the core of the earth is no mere academic exercise. Understanding earthquakes, volcanoes and other geological phenomena depends largely on fathoming the forces at work within the planet's mantle, the thick layer of rock that stretches from the core to within an average of 30 miles of the surface. The behavior of the mantle seems to be determined by the core. The molten center also acts as an electromagnetic dynamo, creating the magnetic field that shields earth from the high-energy particles that stream from the sun.

The new era for earth science began in 1981, when scientists learned that planet-wide vibrations resulting from earthquakes deep within the earth are split into a complex system of overlapping "tones." The implication: there is something going on in the core that no one had previously suspected. Recalls John Woodhouse, a colleague of Dziewonski's at Harvard: "It was the beginning of a new wave of attention to the core."

Before dealing with the core, though, scientists had to understand the intervening mantle, through which all seismic information has to pass on its way to the surface. Explains Dziewonski: "If it's a faulty lens, you're going to have a wrong image." By 1984 the Harvard group had assembled the first detailed map of the mantle ever published. Their data consisted of the patterns of earthquake-generated pressure waves that passed through the solid earth, moving faster through cooler regions of the mantle and more slowly through warmer areas.

Armed for the first time with an accurate picture of the mantle's distorting effects, geophysicists around the world began an intensive probe of the core itself. Using supercomputers, they combined millions of seismological observations collected at some 3,000 surface monitoring stations into a single, overall picture. The image is fuzzy, admits Robert Clayton, a geophysicist at the California Institute of Technology, "but I think everybody now agrees there is some kind of topography down there."