Pluto’s moon Charon might have lost an ocean. And Europa’s and Ganymede’s oceans are pretty old. That a moon could form a new ocean well into its history, and that we could watch?

At Callisto, during 21 flybys JUICE will study its cratered surface, look for evidence of a subsurface ocean and change the angle of its orbit around Jupiter to help it get into orbit of Ganymede.

The measured heights of the tidal peaks in the ice crust, which occur periodically during Ganymede's orbit around Jupiter, will provide clues as to whether there is a liquid ocean beneath the outer ...