Enlarge / Ahuna Mons, a likely cryovolcano. (credit: Dawn Mission, NASA, JPL-Caltech, UCLA, MPS/DLR/IDA) All of the bodies in our Solar System started out hot, with energy built up by their gravitational collapse and subsequent bombardment. Radioactivity then contributed further heating. For a planet like Earth, that has kept the interior hot enough to sustain plate tectonics.