In the Earth but Not of It

By Tim Appenzeller|Wednesday, January 01, 1997
The Earth’s inner core might as well be another planet. This 1,500-mile-wide sphere, made not of rock but of solid iron, is divided from the rest of the planet by the outer core--a moat of churning liquid iron, 1,300 miles thick. In 1996 it was reported that this planet within a planet even spins at its own rate, outpacing the rest of Earth by about a quarter turn per century.

This latest sign of independence wasn’t a complete surprise to Earth scientists. The liquid outer core is so hot that it is no more viscous than water, says seismologist Xiaodong Song of the Lamont-Doherty Earth Observatory, so the inner core should be free to spin on its own, like a beach ball in a bathtub. Moreover, computer models of the outer core, whose convective flow generates Earth’s magnetic field, had suggested that the magnetized liquid might tug on the inner core, twirling it faster than the rest of the planet.

Song and his Lamont colleague Paul Richards decided to test that suggestion with records of seismic waves, which plunge deep into the planet from an earthquake focus before surfacing at distant recording stations. But just as your eye needs the stripes on a beach ball to tell whether it is spinning, Song and Richards needed some marker of direction on the inner core. Fortunately, other seismologists had already found the equivalent of a stripe.

Earthquake waves that travel roughly north-south through the inner core are about 3 percent faster than waves traveling east-west, explains Song, probably because the iron crystals are all oriented roughly north-south. If the inner core is spinning faster than the rest of the planet, he and Richards reasoned, then the relation of that seismic fast track to points on the surface should change over time (unless it happened to be lined up perfectly with the spin axis). As the fast track swung closer to or farther away from some fixed seismic wave path--the path between a frequent source of quakes and a single distant seismometer--waves along that path would move faster or slower.

Nearly 30 years of earthquake records from a seismic station at College, Alaska, showed just such a change. The telltale waves came from dozens of earthquakes between 1967 and 1995 in the South Sandwich Islands, on the farside of the globe near Antarctica. Each quake sent waves through the Earth on many different paths. Waves that penetrated the inner core reached the Alaska station about a third of a second faster in 1995 than they did in 1967, while waves that just missed the inner core made the trip in exactly the same time. The inner core, Song and Richards concluded, is turning, giving waves that pass through it a progressively faster trip. Like the rest of the globe, it spins from west to east, but by about 1 degree faster a year.

Late in the year another pair of geophysicists confirmed the finding. And a computer model of the core developed by Gary Glatzmaier of Los Alamos National Laboratory in New Mexico and Paul Roberts of ucla offered a possible explanation. In their simulations, the churning iron of the outer core organizes itself into two jet streams--deep counterparts to those in the atmosphere--that ring the inner core’s north and south poles. Racing eastward, the jets carry a magnetic field that in turn drags the inner core like the rotor in an electric motor.
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