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The super-hot inner core of the Earth, once thought to be furiously awhirl with motion, might in fact spin much slower than previously suspected (though still faster than the rest of the planet), new research suggests.

The inner core of the Earth , a solid ball of metal roughly 1,500 miles (2,400 kilometers) wide, is thought to reach temperatures as hot as 9,000 degrees Fahrenheit (5,000 degrees Celsius). Although it lies about 3,200 miles (5,200 km) below our feet, it still influences life on Earth's surface as the inner core grows from matter solidifying onto it from the outer core, the heat released during the process drives the flow of metal in the outer core, which generates the planet's magnetic field .

Past research analyzing seismic waves traveling through the planet suggested that the inner core might spin faster than the rest of the planet, gaining one degree of extra rotation a year. Now scientists find that although such "super-rotation" likely exists, it appears extraordinarily slower than previously thought one degree of extra rotation every million years.

Matter solidifies onto the inner core at a rate of roughly 1 millimeter per year, so by scanning through the inner core, scientists can tell how it has changed throughout history. The inner core's eastern hemisphere is better at transmitting seismic waves than its western one, since it loses heat faster, perhaps due to the core's interactions with the Earth's mantle (the layer between the core and the crust on which we live). The boundary between the hemispheres was something researchers saw gradually shift over significant depths and thus long spans of time. This pattern helped the scientists calculate the inner core's rate of spin.

The past estimates that suggested a super-fast inner core can be attributed to short bursts of wobbling of the inner core that clouded up calculations on its spin rate. "The different rates of motion are not incompatible," said researcher Lauren Waszek, a geophysicist at the University of Cambridge in England.

These findings "imply that we should also be able to observe other similarly slow inner core motions," Waszek said. Such research could "lead to a better understanding of the evolution of the Earth's magnetic field," she added.

The scientists detailed their findings online Feb. 20 in the journal Nature Geoscience.