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"Animation of Jupiter's moon Io passing in front of the planet. Io is the innermost of Jupiter's Galilean moons, orbiting some 420, 000 kilometres from Jupiter, although such is the size of Jupiter that this is only 350, 000 kilometres above the tops of the planet's clouds. This is a comparable distance to that between the Earth and the Moon. Io is the fourth-largest moon in the Solar System, at 3600 kilometres in diameter it is just larger than our Moon, and around 20% more massive. Jupiter, however, is some 318 times as massive as Earth. The combination of proximity and this mass means that Io is greatly deformed by Jupiter's immense gravitational pull, and this energy causes heating within the moon. The heating melts the interior of the moon, causing it to be the most volcanically active body in the Solar System."