You might think the length of a day never varies — exactly 24 hours.
You’d be wrong, at least if you count days in milliseconds, as physicists do.
For about 30 years, scientists have known that the rotation of Earth’s liquid core — detected through changes in its magnetic field — speeds up over a few decades, then slows down over a few more. Meanwhile, the mantle — the 3,000-kilometre-thick rocky shell that includes the crust — inversely slows down, then speeds up to compensate, because the angular momentum of the planet must remain constant.
That change in the rotational speed of the mantle is what can shorten or increase the length of a day by a few milliseconds over time.
Understanding the mechanism of exchange between the core and mantle has so far eluded scientists. But in a new study published today in Nature, University of Alberta physics PhD student Huifeng Zhang and professor Mathieu Dumberry show that small changes in the speed of rotation of the inner core — the solid, central part of the core, which is not exactly spherical — exert what’s called a “gravitational torque” on mass anomalies within the mantle, causing a slight fluctuation in the length of a day.
Another form of torque at the boundary of the core and mantle creates friction and electromagnetic drag — called core-mantle boundary torque — resisting the gravitational tug and limiting the length of day changes. The subtle deviations in the balance between the gravitational and core-mantle torque are ultimately responsible for the small, observed changes in the length of day.
In addition to solving this long-standing mystery of the fluctuating length of a day, the research also shines a spotlight on the depths of the Earth, including the inner core, which turns out to “deform viscously,” say Zhang and Dumberry, on a time scale of about 10 years.