* Io: The most volcanically active body in our solar system, Io is a world of sulfurous volcanoes and a constantly changing surface.
* Europa: With a smooth, icy surface, Europa is thought to harbor a vast, salty ocean beneath its shell, potentially making it a prime candidate for extraterrestrial life.
* Ganymede: The largest moon in our solar system, Ganymede is even bigger than the planet Mercury. It has its own magnetic field and a diverse surface of craters, mountains, and vast plains.
* Callisto: The most heavily cratered body in our solar system, Callisto is a frozen world with a very old, ancient surface.
These four moons were first observed by Galileo Galilei in 1610 and are often referred to as the "Galilean Moons".
