Enceladus

Scientists were astonished to see water erupting from a moon as cold as Enceladus: -200 °C. It has yet to be fully explained, but clearly there is heating in the interior. Part of it is the continually changing gravity from Saturn as the moon's distance varies during its orbit, but there must be something else: perhaps radioactive decay. It helps that the water contains salt and ammonia, which lowers its freezing temperature. The Enceladus geysers are the cause of Saturn's E Ring.