The planet’s solid iron inner core has stopped spinning faster than the surface, a new study suggests.
A study carried out at Peking University has found that the centre of the planet is slowing down its rotation. We speak to Dr Jessica Irving, a seismologist and senior lecturer based at the University of Bristol’s School of Earth Sciences about what this means.
What does the inside of the Earth look like?
The crust of the Earth is made of rock. Then we’ve got this huge expanse that we call the mantle. That’s solid rock-like material, but it’s under high pressure and high temperature so it’s different to the rocks that you would find if you wandered out into a park.
Beyond the bottom of the mantle we get into the deepest regions of the Earth, to the core. There we leave the rocks behind and enter a big ball made of iron in the centre. That ended up there because iron is heavier than rock.
We’re talking about a huge ball that’s about half of Earth’s radius made of metal. But we can also split that core into two more distinct chunks.
We have the outer core, which is made of molten metal that’s almost as runny as honey. Then in the middle, we’ve got the solid inner core, which has a radius about a fifth of the Earth’s.
How do we study changes to the Earth’s core?
We have a variety of different techniques to make what we call indirect observations. No hole that has been dug that’s going to help out. The deepest ever hole was a bit over 12 kilometres deep.
To get down to the inner core, we need to go down thousands and thousands of kilometres and we certainly have no samples. Seismologists look at a record of an earthquake wave which has gone right through the rocky mantle – the liquid outer core, into the inner core – and all the way back out and to the far side of the planet.
Then they try and look for another earthquake that happened as close as possible and was detected by exactly the same seismometer.
What causes the core to rotate?
The inner core has a couple of different forces acting on it that could make it spin differently. We’re talking about the core’s differential rotation, the slight difference to what we experience standing on the surface. The Earth’s core is where our magnetic field comes from.
It is known as a geodynamo and is generated by the molten iron moving in Earth’s outer core. The magnetic field extends way out beyond the surface of the planet. You can see it in space. We’re really fortunate to have it there because it protects the Earth from cosmic radiation.
It’s generated in the outer core, and it’s possible that there are twisting or toroidal elements of that field that might actually act on the inner core to twist it slightly. So, they might cause a little bit of differential rotation of the inner core. But that’s not the only thing happening down there.
There are gravitational forces at play as well. The bottom of Earth’s mantle is a bit uneven. Gravity would like these uneven structures to stay lined up relative to the inner core to keep them stable.
But electromagnetic forces would like to tug on the inner core and make it rotate. So we’ve got this huge battle at play right in the centre of our planet between these different forces.
What has the new study found?
It found that when they looked at pairs of earthquakes that occurred as close as possible together at different times in, say, the nineties, they seemed to show some differences in the way the seismic waves went through the inner core. But pairs of earthquakes that were more recent don’t seem to show differences.
So, what the paper essentially says is maybe the inner core rotation is happening at a slightly different speed relative to the rocky bits of the Earth. That’s the big takeaway. It does have some side effects, though. It might be linked to ever so slight changes in the length of a day. And this is one of the conclusions that this paper draws.
How big an effect could that have?
It’s not enough time to have an extra cup of tea in the morning. These are really tiny changes of maybe a 10th of a millisecond in the length of a day.
What can we learn from studies like this?
I want to be clear that there are different scientists studying this. Some of them think the inner core is changing its rotation rate slightly. But what we’re really studying is the dynamics of what happens inside a planet. We know that we didn’t always have an inner core, but we don’t even know how old it is.
It’s currently got a radius about a fifth of the Earth’s, but maybe if you go back a billion years it wasn’t even there at all. It’s been slowly growing over many millions of years. We also don’t understand exactly how the inner core growing could have changed our magnetic field, although we know it must have done.
As scientists, we’re still really trying to understand what goes on in the middle of a planet. Earth is the planet we can study most easily, but scientists are also interested in what’s happening in the core of Mars, for example. There are also missions to study the core of Mercury. We’re at this place where we’re really starting to understand how planets work.
We’d also like to better understand how Earth’s magnetic field works. We know it flips, for example, but we know that isn’t a process that happens like clockwork. It’s really complicated.
So, if we can understand what the inner core is doing, we might be able to understand a bit more about the different sorts of magnetic field processes and that could help us understand how our protective shield works. Any bit of data is really useful.
About our expert, Dr Jessica Irving
Jessica is a seismologist and senior lecturer based at the University of Bristol’s School of Earth Sciences. Her work has been published in the journals Geophysical Journal International, Experimental Astronomy and Nature Astronomy.
Read more about the Earth:
- Why doesn’t Earth’s core melt the planet?
- What is the earliest, geologically, that humans could have survived on Earth?
- The thought experiment: What would happen if the Earth stopped spinning?