Heart of Iron

By Tim Appenzeller|Monday, January 01, 1996
It’s a lump of solid iron about the size of the moon, but it’s far more inaccessible. It’s no farther from any point on Earth than Europe is from the East Coast, but we can go there only in our science fiction novels--and, it now seems, in our computer simulations. This past year geophysicists made a computer-aided visit to Earth’s inner core, simulating the behavior of iron at the crushing pressures that prevail there. At the heart of our planet, they discovered, inside the thick envelope of liquid iron that gives rise to the geomagnetic field, there may be a metallurgical marvel: a single, perfectly ordered crystal of iron, a thousand miles or more across.

Ron Cohen of the Carnegie Institution of Washington and Lars Stixrude of the Georgia Institute of Technology set out to explain an old mystery about the inner core: why seismic sound waves traveling through the planet from earthquakes cross the inner core about 4 percent faster when they’re moving north-south, along the Earth’s spin axis, than when they’re moving parallel to the equator. The difference, most researchers have assumed, reflects some kind of grain in the inner core. By lining up along the spin axis, iron crystals might make the inner core stiffer along the axis, thus making sound travel faster in that direction.

But no one knew what the speed of sound in iron was at inner-core pressures, let alone what particular crystal structure might explain the speed differences observed by seismologists. Cohen and Stixrude decided to calculate those things from first principles. What we’re doing is starting from fundamental physics and quantum mechanics and trying to work up to what the speeds could be in an iron crystal at the center of Earth, says Cohen. After a few hundred hours on a supercomputer, the researchers had an answer.

The best match to the seismologists’ measurements came from a crystal form of iron known as hexagonal close-packed (hcp), in which the atoms are arranged in planes like racked pool balls, each atom touching six others, and the planes of hexagons are stacked up, with alternating planes slightly offset, to give the three-dimensional structure. In a collection of hcp crystals at inner-core pressures, the computer calculations showed, seismic waves would travel faster perpendicular to the hexagonal planes than parallel to them. But this difference was large enough to match the seismic measurements only when the crystals were aligned almost perfectly.

And that, says Cohen, suggests that most of the inner core may be a single giant crystal rather than a mass of tiny ones. Such a crystal, he argues, could easily have grown over the billions of years of Earth history. As our planet grows older and colder, the solid inner core is growing continuously at the expense of the liquid outer core: the liquid is freezing and snowing onto the surface of the solid. As the crystals are buried ever deeper, says Cohen, like old snow sinking into a glacier, the iron might recrystallize into larger crystals. The new crystals would align, taking their direction from Earth’s magnetic field or, Cohen suggests, from the stresses generated by Earth’s rotation. Either way the hexagonal planes would end up more or less perpendicular to the planet’s north-south axis. All the driving forces are in favor of growing essentially one big crystal, says Cohen.

Not all geophysicists are in favor of such a simple picture, though; it’s still just a hypothesis. But if it’s true, it could explain several puzzles about the geomagnetic field, which is generated by turbulent currents of iron in the outer core. The inner core can’t generate a magnetic field on its own, but the magnetism of the outer core induces a field in it, just as a permanent magnet can temporarily magnetize a paper clip. If Cohen and Stixrude are right about the overall alignment of the iron, the inner core would have a single easy axis of magnetization--a direction in which the induced field would be strongest. And if that direction didn’t perfectly match the field from the outer core, Stixrude and Brad Clement of Florida International University have argued, the inner core could skew the field seen at the surface.

Several things are askew about that field. The field lines at the equator don’t run exactly parallel to Earth’s surface: they’re tipped down a few degrees toward the north--perhaps, say Stixrude and Clement, because they’re pulled by the tilted field of the inner core. Every few hundred thousand years, when the north and south magnetic poles trade places, they temporarily get stuck in an intermediate orientation--determined, perhaps, by the inner core. All this is highly speculative, Stixrude and Clement acknowledge. But if you can explain many things at once with a single lump of iron, says Clement, then maybe you’re onto something.
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