Why days change slightly: the role of the Earth's inner core and mantle

• Days change by a few milliseconds
• The liquid core speeds up, the mantle slows down, and vice versa
• The gravitational torque of the inner core affects the mantle
• The balance of gravity and friction limits changes in the length of the day

For most people, a day is perceived as an unchanging 24-hour segment, however, measurements show that the length of Earth days changes by fractions of a millisecond. Such deviations are recorded using high-precision astronomical and geophysical methods that allow tracking how the planet's rotation speed changes.

Over the past thirty years, scientists have discovered that the Earth's liquid outer core rotates at a variable speed. The planet's magnetic field, generated by the movement of this layer, indicates that over several decades the core can speed up and then slow down. At the same time, opposite changes are observed in the mantle – a massive rocky layer about 3000 km thick, which includes the Earth's crust.

Exchange of angular momentum

Such mutual exchange of speed is explained by the law of conservation of the Earth's total angular momentum. When the outer core accelerates, the mantle slightly slows down, and vice versa. Even minor changes in the speed of the mantle can alter the length of a day by a few thousandths of a second.

The exact mechanism of angular momentum transfer between layers has long remained unknown. In a study published on September 23 in the journal Nature, physicists from the University of Alberta – graduate student Huai-feng Zhang and Professor Mathieu Dumoulin – proposed an explanation based on gravitational torque.

Gravitational and magnetic torque

Scientists showed that small changes in the rotation speed of the solid inner core, located at the very center of the planet, create gravitational torque. Since the inner core is not perfectly spherical, its movement interacts with mass inhomogeneities in the mantle, causing slight acceleration or deceleration of the latter.

At the boundary between the core and the mantle, there is a so-called core-mantle boundary torque. It is caused by friction and electromagnetic resistance, which counteract gravitational pull and limit the change in the length of the day.

According to researchers' findings, the observed fluctuations in the length of the day arise from the balance between these two forces: gravitational torque and resistance at the core-mantle boundary. When one of the forces strengthens, the other compensates for its influence, maintaining the planet's overall angular momentum.

In addition to explaining a long-known phenomenon, the results provide insight into the dynamics of the Earth's deepest layers. The authors note that the inner core deforms 'viscoelastically' on decadal timescales, indicating a more flexible and mobile behavior of the planetary core than previously thought.

These findings open up new opportunities for studying the geodynamics of the planet, help refine models of Earth's rotation, and may have practical implications for high-precision navigation systems that depend on accurate timing.

Source: ScienceDaily