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.
Since its discovery centuries ago, Ganymede has been the target of a great deal of well-deserved attention from the planetary science community. Earth-bound telescopes have gazed at Ganymede's puzzling, icy surface and, in later decades, flyby space missions and spacecraft, circling around Jupiter, have scrutinized Ganymede--trying to solve its numerous mysteries. These observations ultimately unveiled a complicated, icy moon-world, whose bizarre surface showed a strange and puzzling contrast between its two main types of terrain: the dark, extremely ancient and heavily cratered surface terrain, and the much younger--but still ancient--lighter terrain showing a vast array of mysterious grooves and ridges.
The Cassini-Huygens mission is a cooperative endeavor by NASA, the European Space Agency (ESA), and the Italian Space Agency. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, California, manages the mission for NASA.