How well do you know the Galilean moons? The Galilean moons are Jupiter’s four largest moons, named for Galileo Galilei, the Italian astronomer who observed them in 1610. Ganymede is the largest moon in our solar system—even bigger than Mercury and Pluto. It’s also the only moon in our solar system with its own magnetic field and the auroras that come with it. Its icy surface is marked by streaky craters; most are bright white, but around 1% have dark streaks. Io is a moon covered in volcanoes and molten lava pools. It is the most volcanically active world in our solar system, with eruptions that can reach dozens of miles high. Some volcanic bursts can even be seen through large telescopes on Earth! Callisto is the most heavily cratered object in our solar system, with its densely dotted surface revealing a long history of impacts, likely from asteroids or comets. Due to the density of these craters, scientists believe this moon has little geologic activity to clear them. Last, bu...