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Discovered by Galileo Galilei in 1610. Callisto's surface is heavily cratered and ancient, a visible record of many impacts over billions of years. It is the most distant of the four from Jupiter.

Discovered by Galileo Galilei in 1610. Io is the most volcanically active body in the Solar System, with over 400 active volcanoes. Its surface is dotted with sulfur compounds which give it a colorful appearance.

Discovered by Galileo Galilei in 1610. Ganymede is the largest moon in the Solar System and even bigger than the planet Mercury. It's the only moon known to have its own magnetosphere.