EARTH'S CORE

The solid-iron core at the centre of our planet is slowing down, according to a new study... and it's making our days longer

- DR JESSICA IRVING

WHAT DOES THE INSIDE OF EARTH LOOK LIKE?

Earth's crust is made of rock. Then going deeper 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. Beneath the mantle, we get into Earth's deepest regions, near the core. There, we leave the rocks behind and enter a world made of metal, specifically iron.

That metal ended up there because iron is heavy compared to rock. So that density contrast has put most of Earth's iron into this big ball at the centre. We're talking about a huge ball that's about half of Earth's radius and 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 roughly as runny as water. Then, in the middle of Earth, we've got the solid inner core, which has a radius about a fifth that of Earth.

HOW DO WE STUDY CHANGES OCCURRING WITHIN EARTH'S CORE?

We have a variety of techniques to make what we call 'indirect observations'. No hole that has been dug is deep enough to help. The deepest-ever hole was slightly over 12km deep. For us to reach the inner core, we'd need to go down thousands and thousands of kilometres, and we certainly have no samples from there.

Seismologists look at a record of an earthquake wave that has passed right through the rocky mantle, the liquid outer core, into the inner core, and then has come all the way back out and onto the far side of the planet.

Then they try and look for another earthquake that happened as close as possible to that first one and was detected by exactly the same seismometer some years later.