Solar System Objects
Galilean moons
The Galilean moons are the four largest satellites of Jupiter—Io, Europa, Ganymede, and Callisto—first observed by Galileo Galilei in January 1610. They exemplify diverse worlds, from Io’s extreme volcanism to Europa’s likely subsurface ocean, Ganymede’s intrinsic magnetic field, and Callisto’s ancient, crater-scarred surface.
Ganymede (moon)
Ganymede is the largest moon in the Solar System and a satellite of Jupiter, notable for being the only moon with an intrinsic magnetic field. Observations from Voyager, Galileo, Hubble, Juno, and the James Webb Space Telescope indicate a differentiated interior, a subsurface ocean, and a tenuous atmosphere with oxygen, water vapor, and carbon dioxide.