Jeffrey Fox, of Ardgowan School, asks :-
Why is the core of the Earth warm?
David Nobes, a geophysicist at the University of Canterbury, responded.
The answer is simple but there is some interesting background.
Over 100 years ago, Lord Kelvin, like others of his day, wondered how old the Earth was. In the early solar system, gravity pulled clumps of matter together. Some clumps formed larger bodies, the planets. Gravitational attraction compressed the planets, and the interiors became quite dense. This `gravitational collapse', as it is called, releases energy. Energy can be neither created nor destroyed, and much of the energy released by gravitational collapse was transformed into heat energy. The planet cools, through heat escaping from the planet's surface. The rate at which the heat escapes depends on how long the process has been going on, ie the age of the planet. Kelvin calculated the Earth's age to be approximately 100 million years. We now know that he was wrong.
The rocks of the Earth are, to a greater or lesser degree, radioactive. The decay of short-lived radioactive elements can be used to diagnose and treat cancers and, like tritium and caesium, to determine how long water has been underground, for example, under the Canterbury plains.
Rocks contain traces of long-lived radioactive elements such as uranium, thorium and potassium. These can be used to determine the age of the Earth, about 4.5 billion years. When radioactive atoms decay, they emit high energy sub-atomic particles which heat the rock. On Earth, radioactive heating is much greater than the remnant of the heat released by gravitational collapse.
So the core of the Earth is hot because of radioactive decay, a fact recognised by Ernest Rutherford, with a little bit of heat left over from the gravitational collapse which formed the planet Earth.