Where does Earth's Internal Energy Come From? *

There are three main sources of heat in the deep earth: (1) heat from when the planet formed and accreted, which has not yet been lost; (2) frictional heating, caused by denser core material sinking to the center of the planet; and (3) heat from the decay of radioactive elements.

Internal sources of energy mean they are coming from within the earth

Explanation:

all eukaryotic cell have mitochondria : )