Day Length Swings Traced to Earth's Inner Core

A day is not quite the 24 hours taught in school. Variations in day length stretch or shrink a day by several milliseconds over decades, and scientists have now named the force behind those swings: the gravitational pull of the planet's own solid inner core.

The result was published Sept. 23 in the journal Nature by Mathieu Dumberry, a physics professor, and Huifeng Zhang, a doctoral student, both at the University of Alberta. Their paper addresses a long-standing gap in geophysics: scientists have known for years that spin moves back and forth between Earth's core and mantle, but the exact force doing the transfer was unsettled. Writing in Nature's News & Views section, geophysicists Dominique Jault and Paolo Personnettaz say the new work traces the variation to gravitational coupling between the inner core and the mantle.

A mystery of Earth's spin

For about three decades, measurements have shown that the liquid outer core rotates faster for a few decades, then slower for a few more, while the rocky mantle compensates by doing the opposite, because the planet's total angular momentum must stay constant. According to the study, those trades in spin shift the length of a day by milliseconds across periods lasting roughly 10 to 70 years.

Several candidate forces had been proposed to move the spin across the core-mantle boundary, about 2,900 kilometres below the surface: magnetic forces, pressure against bumps in the boundary, and gravity from the inner core itself. Zhang and Dumberry put each candidate to the test by combining seismic estimates of how the inner core turns relative to the mantle with models of the core's flow derived from changes in Earth's magnetic field. They then checked each force against the observed record of day-length changes between 1964 and 2019.

A tug-of-war deep underground

Their calculations show the inner core's gravity fits the record far better than the alternatives. The solid inner core, a ball of iron roughly 2,400 kilometres across, is not perfectly spherical. When small changes in its rotation shift it away from gravitational alignment with density irregularities in the mantle, the mismatch generates a torque that speeds or slows the whole mantle, subtly changing how fast the planet turns.

Two other torques push back. Friction and electromagnetic drag at the core-mantle boundary resist the gravitational tug, and the authors found these opposing forces run in the opposite phase to the observed changes, meaning they act to dampen the variation in day length rather than cause it. The paper puts it plainly: "The relative ease with which the gravitational torque can explain the multidecadal [length-of-day changes] suggests that it is the main driver of these changes," the researchers write. "The electromagnetic and topographic torques may play a role, but their opposite phase suggests that they act primarily to resist, not drive, the [length-of-day changes]."

The fit also implies something unexpected about the material at Earth's centre. To reproduce the observed record, the inner core must deform viscously, flowing ever so slightly rather than holding a rigid shape, on a timescale of about a decade, the authors report.

Days as a window into the deep Earth

Physicists do not measure day length with watches; they use atomic clocks precise enough to catch the millisecond drift. Other processes nudge the day too: winds and ocean circulation produce seasonal variations, and the Moon's tidal friction stretches days longer over millions of years. The gravitational-torque effect sits between those scales, and its discovery turns a tiny timing wobble into a probe of places no human can reach. According to phys.org's report on the research, the result offers new clues about the shape and softness of the inner core and the continent-sized structures sitting above it.

Readers can find more physics coverage on our Science desk, from the JWST discovery of heavy elements in galaxies formed soon after the Big Bang to the first direct observation of a quantum jump of sound.

The authors are careful about the limits of the finding: the result stands on the seismic and magnetic models underneath it, and independent checks of those models would strengthen the case. Even so, geophysicists now have a physical mechanism that matches both the size and the timing of the swings recorded over more than half a century, and the planet's deepest layers have a new way of making themselves known.