The Department of Geological Sciences at San Diego State University names extraterrestrial impacts, gravitational contraction and radioactive decay as the three main sources of Earth's internal heat. However, Earth's internal heat is much lower now than it was at the early period of the solar system’s existence.
The Department of Geological Sciences at SDSU explains that the kinetic energy of the collisions of metal, rock and ice fragments converted into heat energy as the fragments impacted with the growing Earth. As the Earth become compact, the gravitational energy was converted into heat. The third source of heat is the decay of radioactive elements that change to more stable forms, releasing heat in the process.