By Richard Monastersky
By eavesdropping on earthquake vibrations passing through the globe's innards, seismologists have discovered that Earth's solid core is a solo dancer, spinning separately from the rest of the planet.
Earth's metallic core consists of a solid iron sphere, about three-quarters of the size of the moon, sitting within an outer shell of roiling liquid iron. Hidden beneath 2,600 kilometers of rock, the core has remained the most cryptic realm of our planet. Some theorists have argued that electromagnetic forces inside Earth should cause the core to spin separately from the planet's outer layers. Others have hypothesized that the core should spin in synchrony with the mantle and the crust.
Xiaodong Song and Paul G. Richards of the Lamont-Doherty Earth Observatory in Palisades, N.Y., have found that the inner core rotates in the same direction as the rest of the planet but about 1 degree per year faster. They report their observation in the July 18 Nature.
The surface of the core, at its equator, is moving about 20 km per year relative to the liquid outer core. "That's 100,000 times faster than the types of motion we normally associate with properties of the solid Earth," says Richards. In contrast, the continents creep across Earth's surface at only a few centimeters per year.
Song and Richards embarked on their research after a computer simulation at Los Alamos (N.M.) National Laboratory predicted that Earth's core should spin 1 degree to 2 degrees faster per year than other parts of the planet.
Gary A. Glatzmaier of Los Alamos says he was surprised that Song and Richards could actually observe the core's rotation. "We were really excited to hear that they had found this," says Glatzmaier.
The Lamont-Doherty scientists could not have detected the core's movement without the help of recent discoveries about inner Earth. Ten years ago, seismologists found that earthquake vibrations passing through the solid core travel at different speeds, depending on their direction--a property called anisotropy. Waves go most slowly in the plane of the equator and most quickly when heading poleward.
To explain the differences in speed, researchers proposed that the iron crystals in the inner core have a distinct orientation, like the grain in wood. Waves moving along this crystalline grain travel faster than those going against it.
In the last 4 years, seismologists have uncovered evidence that the grain is skewed slightly from an exact north-south alignment. The axis of anisotropy--corresponding to the fastest direction for seismic waves--tilts about 10 degrees from Earth's axis of rotation.
Song and Richards realized that this axis of anisotropy, which currently points toward Siberia, should shift over time if the core rotates separately. To test the idea, they pulled out old recordings made in central Alaska of earthquakes that occurred in the South Atlantic. Some of the seismic waves from the southern quakes passed through the inner core on their way north. Comparing the old records with newer earthquake recordings, the scientists determined that seismic waves now take about 0.3 second less to complete the journey than they did during the 1960s. This finding suggests that the core has rotated in a way that speeds seismic waves heading from the South Atlantic to Alaska.
The Lamont-Doherty scientists and other teams of researchers are now examining different seismic records to validate the discovery and to measure the core's rotation with more precision.
"I think this is one of the most exciting things to happen in the last several years in deep-Earth studies," says Kenneth C. Creager of the University of Washington in Seattle.