The discovery adds to a growing club of moons in the solar system with theoretically habitable subglacial oceans.
Water is the only substance ever detected on Mimas, indicating that its entire surface is water ice.
Jostling between the surface and core is the most probable source of the heat that melts the ocean layer.
Re-examining Cassini data also helped scientists learn more about another Saturn moon, Enceladus, last year.
The discovery confirmed that the moon harbors all six elements known to be essential to life.
Mimas has a radius of just 123 miles.
Its surface is covered in craters mostly ranging between 12 and 25 miles in diameter.
Named the Herschel crater after the moon’s discoverer, it covers one-third of Mimas’s diameter.
An impact any larger might have broken the moon apart.
