Something peculiar is going on a long way beneath our feet. The rotation of the Earth's inner core is not aligned with the rotation of the mantle, creating a wobble that affects the motion of the poles and even the length of the days that our planet experiences.
The variations are small – so they are not to blame for how quickly the weekend goes away – but they create measurable effects. Effects that would be hard to explain without some misalignment and motion of the inner core.
The inner core is the very center of our planet, a ball of mostly iron and nickel 2,440 kilometers (1,520 miles) across. Around it, there is the outer core made of molten liquid metal, 2,260 kilometers (1,400 miles) thick. The outer core is the main source of the planet’s magnetic field and sits beneath the thick mantle. The new work suggests that the rotation axis of the inner core is 0.17 degrees off compared to the rotation of the mantle.
The team states that the value is much smaller than the previous assumption of 10 degrees used in some geodynamical models. Interestingly the tilt is currently pointing west, suggesting that the northwestern hemisphere of the inner core might be slightly more dense than the rest. The tilt between the mantle and the core is static; it is not changing over time.
The polar motion and the length-of-day fluctuations were the key data that set the researchers on the hunt for the weird behavior between the core and the mantle. A day is roughly 24 hours, but there are small variations here and there depending on a variety of factors. The atmosphere, the tides, the motion of the continents, and the melting of glaciers are some of the effects that can generate variations.
Since the late 1980s, researchers have suspected that a coupling between the inner core and the mantle could be responsible for a periodic variation on the order of 10 years. This study puts the wobble at 8.5 years, plus or minus 75 days. It was the discovery of a signal in the polar motion back in 2018, combined with the day-length fluctuations, that led the researchers to conclude that they were being caused by the same process, a small misalignment.
The wobble and the misalignment also imply that the outer core and inner core do not just differ in state, one solid and one liquid; there is also a difference in density between the two.
The study is published in Nature Communications.