Most of us view a single day as a fixed, reliable 24 hours. However, Earth's rotation isn't perfectly constant. Scientists have long known that the length of a day fluctuates by a few milliseconds over decades, but explaining why this happens has remained a lingering mystery—until now.
In a study published in Nature, researchers Huifeng Zhang and Mathieu Dumberry from the University of Alberta suggest that subtle variations deep within Earth's inner core are responsible for shifting the mantle's rotation speed.
Core and Mantle: A Rotational Tug-of-War
For nearly 30 years, geophysicists have observed that Earth's liquid outer core speeds up and slows down over decades-long cycles.
Because Earth's total angular momentum must stay balanced, any change in core speed causes an equal and opposite reaction in the rocky mantle above it:
- When the core speeds up, the mantle slows down.
- When the core slows down, the mantle speeds up.
Even a fraction-of-a-percent shift in how fast the mantle rotates is enough to alter the duration of an everyday 24-hour cycle by a few milliseconds.
A Hidden Gravitational Pull Deep Underground
While researchers knew that this momentum swap occurred, the exact force driving it remained unclear. Zhang and Dumberry's work shows that the answer lies in gravitational interaction:
- Non-Spherical Core: Earth's solid inner core is not a perfect sphere.
- Gravitational Torque: As the inner core shifts position, its gravitational pull interacts with unevenly distributed masses within the mantle.
- Mantle Adjustment: This gravitational tug slightly changes the rotational speed of the mantle, directly shifting the length of our day.
A Delicate Balance of Internal Forces
Gravitational torque isn't acting alone. At the core-mantle boundary, opposing forces—such as friction and electromagnetic drag—work against the gravitational pull.
The slight variations we experience in day length are the result of a constant, subtle shift in the balance between these two forces: gravitational torque pulling on the mantle, and core-mantle boundary torque resisting the movement.
A More Dynamic Inner Earth
Beyond solving the mystery of Earth's shifting day length, these findings provide new insights into the planet's deep interior.
The study reveals that the inner core appears to "deform viscously" over roughly 10-year cycles. This indicates that Earth's deepest solid layers are far more dynamic and adaptable than previously thought, changing shape and responding to internal forces thousands of kilometers below the surface.
