Earth's inner core reversed direction and is slowing down, and scientists don't know why

• Scientists have debated for years about what's going on with Earth's inner core.
• A new study offers compelling evidence that the inner core reversed direction and is slowing down. 
• The shift happened in 2010, but it's unclear what triggered it or when the core will change back.

One of the key questions plaguing Earth scientists this past decade is "What's going on down there?"

Below your feet, about 3,400 miles down, is Earth's inner core. It's nearly as large as the moon, as hot as the surface of the sun, and helps maintain the planet's magnetic field that shields us from cell-obliterating cancer-inducing space radiation.

In the last decade, scientists have been getting some unusual data about the inner core's behavior — data that suggests its rotation is going a little haywire.

The data implies that in 2010, the inner core reversed its rotational direction compared to the Earth's surface — a phenomenon called backtracking. Now, the inner core is rotating more slowly than before the shift.

There's no risk of a cataclysmic disaster of dead birds falling from the sky or skin-blistering sunburns in seconds, like in the 2003 blockbuster "The Core." The most we might experience on the surface is a minuscule lengthening in our days as rotation slows, but the change would be so small — we're talking milliseconds — that we probably wouldn't even notice.

New study may settle backtracking debate

Scientists aren't even sure what's really going on down there. It's not like we can crack the planet open and examine it.

Backtracking also hasn't happened in the last 40 years, so the possibility of such a massive object undergoing such an extreme change has been more a topic of debate than a scientific certainty.

But a recent study offers a new way of looking at the data that could help settle the debate. The research team behind the study even goes so far as to say they have the "most definitive evidence" yet that the inner core is, indeed, backtracking and moving more slowly.

"We're showing that it really does happen when about half the community didn't believe any of these studies for a while," John Vidale, a researcher involved with the study and dean's professor of Earth sciences at the University of Southern California, told Business Insider.

Proving the inner core is backtracking

The research team analyzed and compared seismograms from over 100 repeating earthquakes that occurred between 1991 and 2023 in the South Sandwich Islands in the South Atlantic Ocean.

Repeating earthquakes are seismic events with nearly identical magnitudes that occur in almost the exact same location, along the same fault. Seismic energy is one of the few ways we can study the inner core because the energy waves can travel from the surface, through the mantle, to the core, and back again, where scientists can detect and measure it.

Vidale and the team looked at how well the seismograms from repeating earthquakes correlated with one another.

"We can see changes in the waveforms of the seismograms as the inner core moves," Vidale told BI.

Their approach offers "the most definitive evidence so far," that backtracking is happening, the team reported in a paper published June 12 in the peer-reviewed journal Nature.

Typically, scientists measure the time differences between seismic waves and how long it takes them to travel to the core and back. This can help map the core's position and how it changes over time. But it comes with a lot of guesswork around the inner core's structure, "and we don't really know the structure down there that well," Vidale told BI.

The team's new method doesn't require that kind of guesswork because they were simply looking at how well the seismograms matched up.

However, even if we can say with more certainty that the inner core is backtracking and slowing down, it's difficult to calculate an exact speed of rotation or what's causing the shift in the first place.

More likely than not, the inner core's behavior has to do with some sort of drag or friction with the outer core or gravitational influence from Earth's mantle, the researchers reported in the paper.

Whatever the reason, there's still a lot we have to learn about the massive object churning beneath our feet.