One of geology's deepest mysteries, and the source of ongoing debate, is whether the solid core of the Earth rotates. Now, a new analysis of nuclear explosions from the 1970s adds more evidence that the inner core does indeed spin on its own, at about 0.15 degree per year relative to the rest of the Earth--or roughly once every 2400 years.
Earth's inner core is a solid iron sphere about 2500 kilometers across. At 4000°C, it is almost as hot as the surface of the sun, but enormous pressure keeps it solid. The inner core is surrounded by molten iron; convective flows in this liquid layer are believed to create Earth's magnetic fields. If the inner core rotates, it might also influence the magnetic fields.
In 1996, a team led by Xiaodong Song and Paul Richards of Columbia University in New York analyzed seismic waves traveling from one side of the globe to the other after an earthquake. The waves slowed when they passed through the inner core in a way that indicated the core was rotating eastward with a velocity of about 1 degree per year relative to the rest of the Earth. However, observations by other groups suggest a slower rotation, or none at all.
Stepping into the debate, geophysicist John Vidale of the Lawrence Livermore National Laboratory in California and his colleagues tapped a new source of data to probe the core's movements. They analyzed readings from seismographs in Montana used during the Cold War to detect nuclear explosions. The USSR tested two warheads, one in 1971 and one in 1974, at almost the same spot on polar islands north of Russia called Novaya Zemlya. The Montana seismographs, spread out over 50 kilometers, could pinpoint the direction of the blasts' seismic waves deflected by the Earth's core. Based on the arrival times of signature waves, Vidale's team concluded that telltale irregularities in the core had rotated a few kilometers during the time between the two explosions, they report in the 25 May issue of Nature.
This method for tracing the inner core movements is "very promising," says geophysicist Annie Souriau of the French research agency CNRS in Toulouse. However, she says that rotation rates won't be reliable until more data are analyzed and the core's irregularities are better mapped.