LOS ANGELES, Aug. 25 (Xinhuanet) -- Scientists reported on Thursday that Earth's core rotates faster than its surface by about 0.3 to 0. 5 degree per year and this finding is expected to end a nine-year debate.

    The claim that Earth's inner core was getting ahead of itself seemed odd at first. But some computer simulations showed the molten-iron outer core dragging the inner core around by the magnetic field generated in the outer core.

Analysis of nearly identical earthquakes that happened years apart proves Earth's iron core rotates faster than the rest of the planet. (worldbook.com)

    Still, seismologists had problems with measurements of the inner core's excess spin. Now the researchers found persuasive evidence that the inner core really is spinning faster than the rest of the planet.

    "Extraordinary claims require extraordinary proof," said Xiaodong Song, a professor of geology at the University of Illinois at Urbana-Champaign and co-author of a paper to appear in the Aug. 26 issue of the journal Science. "We believe we have that proof."

    Earth's iron core consists of a solid inner core about 2,400 kmin diameter and a fluid outer core about 7,000 km in diameter. The inner core plays an important role in the geodynamics that generates Earth's magnetic field, and an electromagnetic torque from the geodynamics is thought to drive the inner core to rotate relative to the mantle and crust.

    The first observational evidence for differential rotation was presented in 1996. For the past nine years, some seismologists have suspected that flaws, or artifacts, in the data were responsible for the purported movement.

    But by comparing historical seismic waves traversing Earth's fluid and solid cores, the researchers found compelling evidence for differential rotation of the solid inner core.

    They reported observations of 17 sets of similar seismic waves,which are called waveform doublets, from earthquakes occurring in the South Sandwich Islands region off the coast of South America.

    The doublets, which were recorded at up to 58 seismic stations in and near Alaska with a time separation of up to 35 years, allowed the researchers to detect temporal changes along the sampling paths.

    The similar seismic waves that passed through the inner core show systematic changes in travel times, and wave shapes when the two events of the doublet are separated in time by several years. The only plausible explanation is a motion of the inner core, the researchers said.

    The most likely explanation for why the inner core is rotating at a different speed is electromagnetic coupling, according to the scientists.

    The magnetic field generated in the outer core diffuses into the inner core, where it generates an electric current. The interaction of that electric current with the magnetic field causes the inner core to spin, like the armature in an electric motor.

    The fluid outer core decouples the solid inner core's movement from the mantle. Because the fluid outer core is not very viscous,frictional drag is small, the researchers said.

    "Differential rotation is a fundamental dynamic process that goes to the heart of the origin of our planet and how it has evolved," Song said. "There is still much to learn about the inner Earth." Enditem