What keeps the Earth's core so hot?

Chris - It's a combination of things... 

One, the Earth's quite a big planet relative to Mars, which is a bit smaller. There was a lot of heat that was in the Earth to start with. When the planets were first forming around the Sun, in what's called a protoplanetary disc, a lot of the swirling and spinning material was crammed together and squeezed together. It had a lot of heat from that, those frictional effects.

Also the Earth has what's loosely termed as radioactive compounds inside the Earth. As these radioactive compounds break down and decay they produce heat. The heat is obviously concentrated in the core of the Earth and then filters up towards the surface. Because the Earth's a big planet it's got a big core. It's got lots of radioactive decay going on. Some of the heat that we're seeing is because the Earth is sustaining it's own heat by radioactive decay.

Kat - Like having a nuclear reactor in our core...