For the past two decades, the movement of this solid yet searing hot metal sphere, suspended in the liquid outer core, has been studied closely and debated by the scientific community. Past research has shown that the inner core has been rotating slightly faster than the planet’s surface.
But a different picture is emerging under a study led by the University of Southern California and published this week in Nature. The research team, which includes U geology professor Keith Koper, verified with new evidence—built on analyses of seismographic data—that the inner core’s rotation began to ease and synced with Earth’s spin about 14 years ago.
Keith Koper, University of Utah
The inner core is a solid sphere composed of iron and nickel, surrounded by the liquid iron outer core. Roughly the size of Pluto at 2,442 kilometers in diameter, it accounts for only 1% of Earth’s mass, yet it influences the magnetic field enveloping the planet and the length of the day. But the core’s location, more than 3,000 miles below Earth’s surface, presents a challenge to researchers since it can’t be visited or viewed.
Past research into the inner core’s movement has relied on data from repeating earthquakes, which occur in the same location to produce identical seismograms. Differences in the time it takes for the waves to pass through Earth indicate how the core’s position changed during the period between two repeater quakes.
In the latest study, researchers analyzed seismic data associated with 121 earthquakes that occurred in the South Atlantic between 1991 and 2023.
“The inner core is just sitting in this fluid outer core, so it’s decoupled a little bit from the rest of the planet. It’s rotating at a different rate,” Koper said. “The angular momentum has to be conserved, so if it’s rotating differently, then that could affect the rotation observed at Earth’s surface. One of the big ideas in this paper is we have basically a new model or new observations about how the inner core is rotating slightly differently than the rest of the planet.”