Earth's internal heat

First of all, you need to find out the reasons why the temperature increases with depth. On this subject expressed by many different reasons: to explain the point to the chemical processes and the transformation of mechanical work into heat, assumed that the entire solar system passed through the warmer parts of the world space, and finally saw the cause of the phenomenon in the initial heat, which, according to Kant's theory - Laplace, Earth has at its separation from the Sun.