About Jupiter's Moons
By most counts, Jupiter has between 80 and 95 moons, but neither number captures the complexity of the Jovian system of moons, rings and asteroids. The giant planet commands thousands of small objects in its orbit. Scientists are getting so good at spotting tiny moons orbiting distant, giant planets that the International Astronomical Union has decided the smallest will no longer be given mythological names unless they are of “significant” scientific interest.
Jupiter's four largest moons were the first moons discovered beyond Earth. They are called the Galilean satellites after Italian astronomer Galileo Galilei, who first observed them in 1610. The German astronomer Simon Marius claimed to have seen the moons around the same time, but he did not publish his observations and so Galileo is credited with their discovery.
Io is the most volcanically active world in the solar system, with hundreds of volcanoes, some erupting lava fountains dozens of miles (or kilometers) high.
Europa is thought to have an iron core, a rocky mantle and an ocean of salty water that may be one of the best places to look for life beyond Earth in our solar system.
Ganymede is the largest moon in our solar system and the only moon with its own magnetic field.
Callisto is the most heavily cratered object in our solar system.
Four more innermost moons are the the source of the dust that makes up Jupiter’s four faint rings. The rest of Jupiter's moons orbit much farther out from the planet. The larger moons may be captured asteroids and the smaller ones are likely fragments from massive collisions.