"Cassini's seven-plus years... have shown us how beautifully dynamic and unexpected the Saturn system is," commented project scientist Dr. Linda Spilker of NASA's JPL to Time Magazine's online edition on March 23, 2012.
Ganymede: Ganymede is both the largest moon of Jupiter, our Solar System's planetary behemoth, as well as the largest moon in our entire Solar system. Observations of Ganymede by the HST in 2015 suggested the existence of a subsurface saline ocean. This is because patterns in auroral belts and rocking of the magnetic field hinted at the presence of an ocean. It is estimated to be approximately 100 kilometers deep with a surface situated below a crust of 150 kilometers.
As time passed, the region would have cooled down considerably and contracted--thus pulling away from its surroundings and forming fractures akin to the cracks that form in mud as it becomes dry--but on a considerably larger scale.