Hurricanes within Earth's core.

Seismologists made a startling discovery this summer when they found that Earth's inner core spins faster than the rest of the planet (SN: 7/20/96, p. 36). The explanation for these internal gyrations may involve hurricanes of liquid iron swirling in the outer core, a trio of physicists now reports.

Earth's core consists of two parts, an inner sphere of solid iron and an outer shell of liquid iron alloy. Convection currents that stir the outer core are critical to producing the superfast spin of the inner core, say Jonathan M. Aurnou, Daniel Brito, and Peter L. Olson of Johns Hopkins University in Baltimore.

In the Nov. 15 Geophysical Research Letters, the scientists describe a simple theoretical model of the outer core's flow patterns that can explain the inner core's spin. In the model, Earth's rotation segregates the outer core's fluid into two regions separated by a cylindrical barrier enclosing the inner core. Cooler liquid congregates outside the cylinder, and warmer alloy gathers in the polar regions within the cylinder.

Buoyed by its warmth, the fluid inside the cylinder rises and starts to spin like the hot air in tropical storms. "You get what amounts to hurricanelike structures," says Olson.

Rotating faster than the rest of the planet, these vortices of iron-rich fluid generate an electromagnetic field that tugs on the inner core, speeding it up, the scientists suggest. Their model would explain why giant computer simula- tions also show the core's superfast spin (SN: 10/19/96, p. 250).

Other scientists advocate a different theory. The spin of the entire Earth, they note, is slowing because of friction from lunar tides. It takes time for the deceleration to pass through the fluid outer core, so the inner core's slowdown may lag behind that of the rest of the planet. The Johns Hopkins team finds that the electromagnetic tug is a much stronger effect, however, and should control the inner core's motion.