Journey to The Earth's Core

By MICHAEL D. LEMONICK

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."

The Harvard group found, for example, that pressure waves travel more quickly when moving parallel to the earth's axis than when they are perpendicular. That could be explained if the solid inner core were a crystal, in which waves would travel at different speeds along different axes, but molten iron is hardly crystalline. Instead, Don Anderson and his colleagues at Caltech's seismological lab postulated the existence of iron rain. Their theory: the polar regions of the core are slightly flattened and tend to be cooler than the equatorial regions. The heat exchange between the two areas may then result in a kind of geological weather system in which iron particles precipitate out of solution and rain down toward the core in a continuous cycle that is comparable to evaporation and condensation.

That scenario could illuminate more than just the confusing differences in the velocity of pressure waves; it could also help explain the mystery of the origin of the earth's magnetic field, as well as its unexplained reversals. About once every million years, magnetic north and south inexplicably exchange places. Scientists do not understand whether this phenomenon comes about gradually, say, over thousands of years, or all at once. One idea, advanced in recent years, is that turbulent eddies within the core-mantle boundary somehow give rise to electromagnetic disturbances that trigger the reversals. A rain of iron particles, say some scientists, might supply the energy to keep the eddies churning.

For all the drama of the recent discoveries, scientists still have a very tenuous understanding of the structure and dynamics of the core. Nonetheless, other areas of geology have advanced enough to give scientists a reasonably consistent idea of how the overall picture fits together. Says Subir Banerjee, a geophysicist at the University of Minnesota: "In every discipline, our measuring capabilities have gone up so much that we are at last able to home in on the earth's core by a number of techniques." For scientists who have long struggled to penetrate the mysteries at the center of the earth, solving the puzzle now seems within reach.

With reporting by Robert Buderi/Boston and Edwin M. Reingold/Los Angeles