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One of _____'s moons is covered with cracked ice. Another one is covered with active volcanoes.

Explanation
Jupiter's four largest moons were the first heavenly bodies in our solar system to be discovered using a telescope. They are called the Galilean moons because Galileo discovered them in 1610. They are Io, Europa, Ganymede, and Callisto. Io is covered with active volcanoes, while some of the plant's other moons are covered with ice.