The heat that remains inside the Earth from its creation and radioactive decay are its main heat sources.
The deep earth receives heat from three main sources: (1) heat left over from the planet's formation and accretion, (2) frictional heating brought on by denser core material sinking to the planet's centre, and (3) heat from radioactive element decay.
The energy produced when unstable atoms decay is known as radioactivity, and it is a substantial contributor to the Earth's heat. The main source are the radioactive isotopes of uranium-235 (235U), uranium-238 (238U), potassium-40 (40K), and thorium-232 (232Th) in the Earth's mantle.
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Answer:
Mercury is the smallest and closet to the sun.
