Final answer:
Earth's internal energy originates mainly from the heat remaining from its formation and the heat produced by the decay of radioactive isotopes within the planet's interior.
Step-by-step explanation:
The Earth gets most of its internal energy from two primary sources: the residual heat from when the planet formed, and the ongoing decay of radioactive isotopes within its interior. Initially, as the Earth formed, gravitational potential energy was converted into thermal energy, contributing to the planet's internal heat. Additionally, radioactive decay, primarily from isotopes of uranium (238U), thorium (232Th), and potassium (40K), continuously generates heat within the Earth's core and mantle.
It is estimated that around half of the Earth's current internal heat budget comes from this radioactive decay. This process produces approximately 44 terawatts (TW) of geothermal power, which flows through the Earth's crust. The contribution of such nuclear reactions to Earth's internal heat budget is significant, even when compared to the vast amount of energy Earth receives from the Sun's radiation.