Earth’s Inner Core May Be Secretly Changing the Length of Our Days

A new study published in Nature suggests that Earth’s inner core may be changing the length of our days by several milliseconds over decades. Researchers found that the inner core's shifting rotation generates a gravitational force against density irregularities in the mantle, producing a tug powerful enough to alter the planet’s rotation over a roughly 70-year cycle. The study shows that the inner core's irregular boundary creates a gravitational torque that matches the observed multidecadal changes in the length of day. The torque is strongest when the inner core rotates in a specific direction, and it alternates in direction around 2010. The model suggests the core-flow pattern switched between two broad states centered around 1967 and 2002, with flows closer to the planet’s axis moving opposite those farther away, alternately displacing the inner core and reversing the gravitational pull on the mantle.

The study’s findings indicate that the inner core is not a perfect sphere and its boundary contains subtle irregularities, giving it a slightly elongated orientation. Density changes in the mantle create an uneven gravitational field, and under normal conditions, gravity tends to keep the inner core’s long axis aligned with those mantle structures. However, flows in the surrounding liquid metal can rotate the inner core slightly east or west relative to the mantle, producing gravitational torque, the rotational equivalent of a pulling force, as gravity tries to restore alignment. The researchers combined seismic reconstructions of inner-core rotation with models of outer-core flows derived from changes in Earth’s magnetic field. They compared the resulting torque with corrected length-of-day records from 1964 through 2019. The inner core appears to oscillate relative to the mantle by about 2.35 degrees, with its rotation changing direction around 2010. Once the researchers allowed its uneven boundary to deform, the ensuing gravitational torque reproduced both the timing and extent of the observed multidecadal variations.

The study also suggests that the inner core’s irregular surface must gradually deform toward gravitational equilibrium over approximately two to 31 years, with the most likely estimates falling near eight to 10 years. That points to a relatively low-viscosity inner core whose outermost portion may be considerably softer than previously assumed. The torque strength also constrains structures at the base of the mantle. The calculations imply that broad variations in the core-mantle boundary’s gravitational topography may rise and fall by only about 102 to 272 feet. Those values support models in which the enormous low-velocity regions beneath Africa and the Pacific, often described as continent-sized “blobs,” contain chemically distinct material that is hotter but intrinsically denser than its surroundings. The opposing effects could make the structures nearly neutrally buoyant. If electromagnetic drag provides much of the resisting force, the results also support a globally distributed, electrically conductive layer of iron-enriched material about 1.2 miles thick at the mantle’s base. The researchers caution that their conclusions depend on reconstructions of inner-core motion and liquid-core flows that contain significant uncertainties. Different flow models shifted some of their torque estimates by as much as 30 percent. Their analysis also explains the broad multidecadal signal better than shorter fluctuations lasting 10 to 30 years, which may involve core-mantle forces that present models do not yet capture.

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