By a nose. Seismic waves from a later quake (white, bottom) follow the same path but arrive sooner because the inner core has rotated more than the rest of the planet.

Spinning Right Around

Nine years after making a controversial claim that Earth's inner core was getting ahead of itself, a team of scientists has new evidence that the inner core really is spinning faster than the rest of the planet.

The claim seemed odd at first. Why should a 2440-kilometer solid iron ball spin faster than the 3000-kilometer-thick shell of mantle rock wrapped around it? Evidence for the faster rotation lay in computer simulations that showed the outer core's magnetic field dragging the inner core around, but measurements of that excess spin proved problematic.

To solve this conundrum, seismologists Jian Zhang and Paul Richards of Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, and Xiaodong Song and Yingchun Li of the University of Illinois, Urbana-Champaign, took another look at the data and reduced two sources of error from their original estimate. First, by determining the exact location of earthquakes near the South Sandwich Islands in the South Atlantic Ocean, the researchers shored up measurements of the time it took seismic waves from these quakes to travel through the core to a seismograph in Alaska. If the inner core rotates faster than the mantle, South Sandwich quakes striking the same place years or decades apart will send out waves that take slightly different paths through the core and arrive in Alaska a little sooner than they did the time before. The researchers found that this was indeed the case.

Second, by combining the more precise travel times with new measurements mapping the uneven woodlike grain of the inner core, Zhang and colleagues were able to calculate a "superrotation" of 0.3° to 0.5° per year, or about 900 years for the inner core to gain one full revolution on the rest of the planet, they report in the 26 August issue of Science . That's about a third as fast as Song and Richards's initial estimate of 1996.

Seismologists are generally impressed. "This paper removes any lingering doubt as to whether the inner core is rotating at a different rate than the mantle," says seismologist Kenneth Creager of the University of Washington, Seattle. Now researchers can consider what these new estimates of superrotation say about Earth's interior. For example, the estimates might help test computer simulations of how the outer core generates the magnetic field, says geophysicist Bruce Buffett of the University of Chicago, Illinois. That's a lot for a little extra spin.