Io (moon)

Innermost Galilean moon of Jupiter

Io is the innermost and second smallest of the four Galilean moons of Jupiter. Slightly larger than Earth's Moon, Io is the fourth-largest natural satellite in the Solar System, has the highest density and strongest surface gravity of any natural satellite, and the lowest amount of water by atomic ratio of any known astronomical object in the Solar System. With over 400 active volcanoes, Io is the most geologically active object in the Solar System.

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