Earth's core out of kilter.

The solid iron core at the planet's heart has an unexpected tilt, according to seismologists who have studied the waves of 15,722 earthquakes that passed through Earths deep interior.

In the mid-1980s, scientists from Harvard University first noticed an unusual feature of Earth's core: Seismic waves tended to travel fastest when they paralleled Earth's axis of rotation. Their speed dropped by as much as 3 percent when the waves moved perpendicular to the rotation axis. The seismologists who discovered this asymmetry explained it by suggesting that the iron crystals in the core point toward the poles and thus transmit seismic waves fastest when they travel that way. This pattern may develop from the way Earth's magnetic field orients the crystals that solidify on the surface of the inner core.

Wei-jia Su and Adam Dziewonski of Harvard now report that the asymmetrical pattern doesn't line up exactly with the rotation axis; rather, it leans 12.5 * from that line. Seismic waves, they found, move fastest when heading to or from Siberia instead of toward a pole. That pattern leads Su and Dziewonski to wonder if most of the core's iron crystals point in that skewed direction.

Because detecting asymmetries in the core is a difficult venture, the new results may well encounter some skepticism. But if such findings hold up in future studies, this tilted pattern could give scientists a much-needed clue to understanding Earth's solid iron center. "The inner core is still a mystery," notes Dziewonski.

If the magnetic field orients the iron crystals as the core grows, then the crystals' tilt would also offer insight into the way the field has changed with time. "By mapping this [pattern], we may be mapping the history of the magnetic field and how the field has existed in the inner core," Dziewonski says.