Earth's Day Lengthens by Milliseconds
Subtle shifts in Earth's inner core cause variations in day length, with the mantle responding in the opposite direction to changes in the core's rotation speed
Earth's day is not exactly 24 hours. The length of a day changes by a few milliseconds over decades, and scientists have discovered that subtle shifts in Earth's inner core may cause these variations.
The reason for this phenomenon lies in the exchange of momentum between the core and the mantle. Earth's total angular momentum must remain constant, and the mantle responds in the opposite direction to changes in the core's rotation speed. When the core speeds up, the mantle slows slightly, and when the core slows, the mantle speeds up.
A study published on September 23 in Nature presents a possible explanation for how the core and mantle exchange rotational momentum. The study, conducted by University of Alberta physics PhD student Huifeng Zhang and professor Mathieu Dumberry, found that small changes in the rotational speed of Earth's inner core can create a gravitational torque that interacts with uneven distributions of mass in the mantle.
This gravitational torque can slightly alter how quickly the mantle rotates, producing small changes in the length of a day. A force known as core mantle boundary torque produces friction and electromagnetic drag that opposes the gravitational pull.
## Why it matters The discovery of the exchange of momentum between the core and mantle provides new insights into the dynamic world thousands of kilometers beneath our feet. The subtle shifts in the inner core and the response of the mantle have significant implications for our understanding of Earth's rotation and the length of a day.
The study's findings are the result of roughly three decades of research into Earth's liquid core and its rotation. Scientists have known that the liquid core does not always rotate at the same rate, and changes in the planet's magnetic field have revealed that the liquid core can gradually speed up over several decades, then slow down during the following decades.
The new study provides a possible answer to the long-standing question of how the core and mantle exchange rotational momentum, and it has significant implications for our understanding of Earth's internal dynamics.
