Earth’s inner core may be deforming within years — scientists say its hidden movements could change the length of our days

The day is something which appears to be very concrete among all objects in this world. Midnight comes right after 11:59; time moves for 24 hours, and then the process repeats itself. However, the Earth itself doesn’t revolve uniformly.

The length of an Earth day can vary by a few milliseconds over periods of decades. The change is far too small for people to notice in everyday life, but precise measurements of Earth’s rotation can detect it. Scientists have long suspected that processes deep inside the planet are responsible for these fluctuations.

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A new study published in Nature offers a detailed explanation for how that may happen.

Researchers from the University of Alberta found that gravitational forces linked to the movement of Earth’s solid inner core could be the main driver of multidecadal changes in the length of a day. Electromagnetic and mechanical forces at the boundary between the core and mantle appear to work against that gravitational effect.

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The study also points to something less obvious: Earth’s inner core may not behave like a completely rigid ball. The researchers’ model suggests that it can deform over timescales of only a few years, with the study indicating a low-viscosity inner core.

That gives scientists another way to investigate a region that sits thousands of kilometres below the surface and cannot be observed directly.

Earth’s 24-hour day is not perfectly fixed

The commonly known 24-hour day is an average value, and it does not remain constant because of the rotation of the Earth.

Earth’s rotation may slow down or speed up. If the Earth slows down a little bit, the day gets longer, and if it speeds up, then the day becomes shorter.

Scientists track them using highly precise observations of Earth's orientation relative to distant astronomical objects. Those measurements reveal subtle variations that would otherwise be impossible to detect.

The puzzle has been figuring out what causes those changes.

Earth is not a single rigid body rotating at one uniform rate. Its mantle, liquid outer core and solid inner core can interact with one another and exchange angular momentum.

The mantle is the thick rocky layer surrounding the core and includes the crust at its outer surface. Beneath it lies the fluid outer core, which is largely made of molten iron and generates Earth's magnetic field. At the centre is the solid inner core.

Because these layers interact, a change in one part of the planet can influence another.

The new study focuses particularly on the relationship between the inner core and mantle, and how gravity can transfer rotational momentum between them.

A gravitational tug inside Earth can affect the clock

Huifeng Zhang and Mathieu Dumberry of the University of Alberta developed a model examining several forces that can transfer angular momentum between Earth's interior layers.

Their analysis suggests that gravitational torque plays the leading role in explaining the observed multidecadal changes in day length. A torque is essentially a twisting force capable of changing rotational motion.

The inner core is not a perfectly uniform sphere. Its uneven distribution of dense material can interact gravitationally with variations in density within the mantle.

That creates a subtle connection between regions separated by thousands of kilometres.

The model suggests that dense areas in the inner core tend to be pulled toward corresponding dense regions in the mantle. But the inner core is also affected by the motion of the liquid outer core surrounding it. The flow of that molten material can push the inner core away from its preferred alignment.

The result is a slow geophysical tug-of-war.

The force of gravity aligns the inner core with the mantle, but the outer core's associated force may act on the inner core to push it in a different direction. This movement of the inner core causes a gravitational effect that transfers angular momentum.

That, in turn, can alter the length of a day by a few milliseconds.

The researchers' calculations found that a model dominated by gravitational torque was able to reproduce important features of the observed multidecadal variations in Earth's rotation.

The inner core may not be as rigid as it sounds

One of the more intriguing implications of the study concerns the physical behaviour of Earth's deepest interior.

The inner core is solid, but “solid” does not necessarily mean completely immovable or rigid over long periods. Under the immense pressure and temperature found at Earth's centre, material can respond to forces gradually.

The researchers' results are consistent with a low-viscosity inner core that can deform over a timescale of only a few years. The study reports a characteristic timescale of roughly 10 years in its interpretation, although the broader model permits a range of timescales.

This matters because scientists cannot drill anywhere close to Earth's centre. Much of what is known about the inner core comes from indirect evidence, particularly seismic waves generated by earthquakes.

A previous study using seismic observations reconstructed changes in the inner core's rotation relative to the rest of the planet. That record showed the inner core rotating somewhat faster relative to the rest of Earth until around 2010, after which its relative rotation slowed. The new work uses that reconstruction as an important part of its model.

By combining seismic observations with measurements of Earth's rotation and models of the forces acting inside the planet, researchers can test ideas about structures that cannot be observed directly.

The changing length of a day therefore becomes more than a measurement of time. It can act as a signal from Earth's deep interior.

Earth’s core is caught between competing forces

Gravity is not the only force involved.

At the boundary between Earth's liquid outer core and rocky mantle, electromagnetic interactions and mechanical effects can also transfer angular momentum.

The outer core is electrically conductive and constantly moving. Those flows generate Earth's magnetic field and can interact electromagnetically with surrounding structures.

There are also mechanical or topographic effects at the core–mantle boundary. Irregularities at that boundary can allow the moving outer core to exert forces on the mantle.

The new model suggests that these effects oppose the gravitational torque generated by the inner core. The resulting balance helps explain why Earth's rotation changes in such a subtle, complicated way rather than simply speeding up or slowing down in response to one process.

The study also has implications beyond the length of a day.

Because the model relies on particular assumptions about Earth's deep interior, its success in reproducing rotational observations provides clues about the properties of the lowermost mantle and the core–mantle boundary.

The researchers' analysis supports the possibility of unusually low-viscosity material and highly conductive structures near the base of the mantle, as well as large dense structures within the deep mantle.

These structures are themselves subjects of ongoing research because scientists are still working to understand their composition, origin and evolution.

The findings do not mean scientists now have a complete picture of Earth's interior. Other factors also influence the planet's rotation, including processes involving the atmosphere and oceans. Independent researchers cited by Nature also noted that uncertainties remain in the seismic reconstruction of inner-core rotation.

Instead, the study adds another piece to a complicated picture of how Earth's layers interact.

And it offers a striking reminder that the planet beneath our feet is not simply a static collection of solid layers. Deep inside Earth, material is moving, forces are competing and rotational momentum is constantly being redistributed.

The result can be detected in something as ordinary as the length of a day — even when the change amounts to only a few milliseconds.

Frequently asked questions

1. Does Earth's day have 24 hours?

Not exactly. The 24-hour day is only a common measurement, while the rotation of Earth differs from it by some degree. Due to fluctuations in the internal structure and the rest of the planet, days may be a few milliseconds longer or shorter.

2. What is changing inside Earth?

The new study focuses on interactions involving the solid inner core, liquid outer core and rocky mantle. It suggests that gravitational torque associated with the inner core is the main driver of multidecadal changes in Earth's rotation, while electromagnetic and mechanical forces oppose it.

3. Can Earth's inner core really change shape?

The study's modelling suggests that the inner core has relatively low viscosity and can deform over timescales of a few years. The researchers' results are consistent with deformation occurring on roughly decade-long timescales.

4. Does the changing length of a day affect everyday life?

The variations discussed in the study are extremely small — generally only a few milliseconds over decades. They do not affect the way we experience an ordinary day. The significance of time measurement lies in science because accurate calculations of the Earth's rotation give us information about processes within the Earth.