As the earth turns, the center of the earth turns even faster.
Confirming assertions first made in 1996, a team of geophysicists are presenting data in the journal Science today showing that the earth’s inner core, a ball of solid iron larger than the moon, spins faster than the rest of the planet. Over a period of 700 to 1,200 years, the inner core appears to make one full extra spin.
That extra spin could give scientists information about how the earth generates its magnetic field.
The inner core, 1,500 miles wide, sits at the center of the planet, ensconced in a sea of hot liquid metal known as the outer core. With nothing to hold it in place, the inner core can rotate independently. Nearly a decade ago, two scientists at the Lamont-Doherty Earth Observatory of Columbia University said it did just that.
Other scientists, however, questioned the analysis, which was based on the speed of earthquake waves passing through the earth. Subsequent attempts to pin down the inner core’s rate of spinning produced a wide range of answers. Some said it spun, but at a much slower rate than the Columbia scientists claimed. Others said they could find no sign that the core was out of step with the other parts of the planet. Some said it seemed to be spinning at a slower rate, not faster.
The same researchers who made the original claim, Paul G. Richards and Xiaodong Song, now a professor of geology at the University of Illinois, led the new research, which they said should remove any remaining doubts.
While it does not precisely pin down how much more quickly the core is spinning, Dr. Song said, “what this particular paper shows is it cannot be zero.”
Gary A. Glatzmaier, a professor of earth sciences at University of California, Santa Cruz, who was not connected with the research said, “Now, most people looking at this data would say, ‘Yes, it is probably rotating faster than the surface of the earth.’ ”
Over the course of a day, the earth spins around once, or 360 degrees. The new research indicates that over a year, the inner core spins an extra 0.3 to 0.5 degrees compared with the rest of the planet.
Uncertainty clouded the 1996 research, which found a rotation rate of 1.1 degrees per year, because Dr. Richards and Dr. Song had to compare seismic signals from different earthquakes in different locations.
The new research is more precise, because the researchers were able to find pairs of moderate-size earthquakes near the South Sandwich Islands in the South Atlantic that occurred years apart, yet shook the same ground in a nearly identical pattern. The seismic waves of each pair of earthquakes were the same when they started out, but changed as they traveled through the earth to Alaska, indicating that something down deep had changed in the interim.
“You just look at these seismograms and something is different,” Dr. Richards said.
While the inner core is almost spherical in shape, its composition appears to have a wood-grain-like layering, which could speed or slow seismic waves.
Scientists believe they understand why the inner core might rotate at a different rate. The flow of rising and falling iron in the liquid outer core generates electric and magnetic fields, which push on the metallic inner core. “The thing is acting like a huge rotor in an electric motor,” Dr. Richards said. “Except this one is running a billion amps.”
Dr. Glatzmaier, of U.C. Santa Cruz, said that computer simulations that he and others had done predicted that the inner core would spin faster, but the models lacked enough details to say how much faster. Now the new data will improve the computer models and give a better idea of how the interior of the Earth works.
“It’s nice when new findings like this come out that reduce the uncertainty in one area so you can learn something,” Dr. Glatzmaier said.
By Kenneth Chang