The Center Spins

A couple of years ago we learned we might need to reevaluate the layout of the planet that we learn in grade school. Instead of the quartet of crust, mantle, liquid outer core, and solid inner core, researchers used waves from earthquakes to determine the inner core might actually be comprised of two distinct sections. 

It’s hard to believe large-scale discoveries about the planet’s makeup still linger out there, but 2023 seems to have brought another seismic shift to our macro viewpoint of Earth.

The old layout of the planet by layer - image by BBC

Our pale blue dot rotates one time approximately every 23 hours, 56 minutes, and 4 seconds. With a circumference of just over 40,000 kilometers, this timing means a spot on the surface of the planet spins at a speed of 460 meters per second. That figure translates to about 1,000 miles per hour. In reality, these figures are relative to your spot on the planet, as the equator spins faster than any other latitude; if one could stand exactly on a pole, you wouldn’t be spinning at all. Still, each point on the planet other than the poles rotates once per day to be in the same position.

Imagine a round orange. Spin it like the Earth. The points on the skin of the orange all return to the same spot after one rotation. The same is also true of any point inside the orange. A seed inside will rotate to the same point after the same amount of time it takes for a point on the skin to do so.

Makes sense.

So, it would follow that a spot on the crust, the mantle, the outer core, and the inner core would all rotate in the same fashion as the planet spins.

In 1996, a group of scientists made a discovery that shook geologists.

A graphic showing how earthquakes allow us to study parts of the Earth we cannot sample directly - image by Jsobe

Using seismic waves from more than three decades of temblors at the same locations, the scientists presented evidence that the inner core actually rotates faster than the rest of the planet. 

Researchers had actually theorized this phenomenon – now called super-rotation – before the seismic data supported it because of subtle oddities in the Earth’s magnetic field. Scientists had noted a “westward drift” of the field for decades. The entire magnetosphere comes from the spinning nickel and iron in the liquid outer core, so they used some fancy mathematics to postulate that the core might be spinning faster than the mantle above.

Forgive the combination of pun and cliche, but how on Earth can this anomaly occur? Scientists believe the liquid outer core acts as a buffer between the inner core and the mantle, freeing up the solid core to potentially spin at a different rate compared to the mantle. A prevalent theory suggests that a thermal “wind” produced in the outer core induces a circulation pattern within the core and the magnetic fields created there produce a magnetic “torque.” The mathematics work out that if the inner core’s viscosity is below a certain threshold, the aforementioned forces might cause super-rotation.

A graphic displaying how the cores of the planet generate a magnetic field - image by Andrew Z. Colvin

Initial estimates placed super-rotation at a difference of 3 degrees per year. However, data collected in the past three decades peg the number as much smaller, perhaps 0.5 degrees or 0.1 degrees per year.

Though the data seem supportive, some scientists don’t believe super-rotation is constant. Instead, they think faster rotation might occasionally occur. Others dismiss super-rotation completely, believing the seismic data come from the changing shape of the core.

As bizarre as it is that a portion of the planet spins at a different rate, that’s not the weirdest part about today’s exploration.

This week, seismologists at Peking University reported that in 2009 the inner core stopped rotating faster than the other portions. Instead, it’s now rotating more slowly than the mantle, which we call subrotation.

The authors theorize that the inner core’s rotation may vacillate during 70-year periods and may have done so since time immemorial. Why? We don’t really know. Why does it matter? The magnetic field will change depending on the specifics of the rotations in the core. The magnetic field is obviously very important, as it protects us from being obliterated by the sun’s death rays. It also affects how fast the planet spins, which means it can change the length of a day!

Some of the early reports about this discovery stated the planet’s core had “stopped rotating” and “changed direction.” If those statements were true, we would be discussing something gargantuan on a universal scale. The confusion stemmed from the language scientists use in these situations. “Stopped” refers to matching speed with the mantle, while “reversing direction” implies a change from super-rotation to subrotation. Sloppy diction, scientists! In reality, of course, the inner core is still spinning in the same direction and it never stopped. It has slowed to the point that it is now supposedly in subrotation.

We’re far from definitives when it comes to the core’s rotation. It will likely continue to oscillate between faster speeds and slower speeds. Only time and more earthquakes will tell!

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