"Extraordinary claims require extraordinary proof," said Xiaodong Song, a professor of geology at the University of Illinois at Urbana-Champaign and corresponding 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 kilometers in diameter and a fluid outer core about 7,000 kilometers in diameter. The inner core plays an important role in the geodynamo that generates Earth's magnetic field, and an electromagnetic torque from the geodynamo 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 by Song and Paul Richards, a seismologist at the Lamont-Doherty Earth Observatory of Columbia University. For the past nine years, some seismologists have suspected that flaws, or artifacts, in the data were responsible for the purported movement.
By comparing historical seismic waves traversing Earth's fluid and solid cores, Song and his colleagues found compelling evidence for differential rotation of the solid inner core. The researchers reported observations of 17 sets of similar seismic waves - 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," Song said. "The only plausible explanation is a motion of the inner core."
The most likely explanation for why the inner core is rotating at a different speed, Song said, is electromagnetic coupling. "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.
"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."
In addition to Song and Richards, co-authors are Illinois graduate students Yingchun Li and Xinlei Sun and Columbia graduate student Jian Zhang and research scientist Felix Waldhauser. The work was funded by the U.S. National Science Foundation and the Natural Science Foundation of China.
Editor's note: To reach Xiaodong Song, call 217-333-1841; e-mail: email@example.com.