Radioactive decay generates nearly half of the heat inside the Earth.

Earth's internal heat budget is estimated at approximately 47 terawatts. A 2011 study by the KamLAND collaboration in Japan used geoneutrino detectors to determine that radioactive decay accounts for about 20 terawatts of that total. The primary isotopes responsible for this heat are Uranium-238, Thorium-232, and Potassium-40.The remaining heat is primordial, meaning it is left over from the planet's violent birth 4.5 billion years ago. This includes heat from the accretion of dust and gas and the friction caused by heavy metals sinking to form the core. This combined thermal energy maintains the outer core in a liquid state, which is essential for the geodynamo process.The geodynamo creates Earth's magnetic field by circulating molten iron. If the radioactive 'fuel' in the mantle and crust were to run out, the Earth would eventually cool and become geologically dead like Mars. This would lead to the loss of our atmosphere as solar winds would no longer be deflected by a magnetic shield.