How does the Earth's core stay hot?

Kat Arney put this to Professor Marian Holness, geologist from the University of Cambridge...

Kat - So Marian, you are our Earth expert. What's going on here? Why does the core keep its temperature or does it?

Marian - It doesn't. it is actually cooling and it has been, all the way through Earth history. It's been doing it very slowly because it's in the centre of the Earth and all that heat has got to get out through thousands of kilometres of solid rock. What's interesting is that because it's cooling down, it started out all liquid. So a great big globe of liquid iron and nickel as Jan says. As it cooled down, it started to solidify. So, in the centre of the Earth now, we have an inner core which is about a thousand kilometres in diameter which is solid and then the outer core is liquid. It's the movement of this outer core in fact that makes the Earth's magnetic field. But what we can do is use the fact that it's solidifying to work out how hot it is because what we can do is we know how big the core is because we can listen to earthquakes and find out where all these different divisions of the Earth are. The division between the solid and the liquid core is about 5,000 kilometres below the surface of the Earth. So we can work out how much pressure there is there and then we do some experiments and say, "When does iron solidify at these sorts of pressures?" And the answer is, 6,000 degrees Celsius.

Chris - Andrew.

Andrew - I was just wondering, does radioactive decay in the Earth contribute to that heating? Does that put some heat back in if you like as well?

Marian - Yes, it does. The Earth started out hot because of all the potential energy that was released when you made the planet in the first place.

Kat - That's everything just like slamming together.

Marian - That's everything slamming together, yes, under gravity. But it's actually cooling much slower than you'd expect if it just had that original heat because as Andrew says, we're generating heat radiogenically by the decay of isotopes like uranium, thorium, lead, potassium, some early isotopes, and tungsten isotopes that are now completely dead, so there's nothing left of them. But in the core, you don't really have that. You don't really have radiogenic heat generation in the core. It's all in the outer mantle, the rocky bit.

Kat - Absolutely fascinating! Thank you.