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The innermost of the four Galilean Moons, orbiting Jupiter at a distance of some 421,800 kilometres. Io is only slightly larger than Earth's Moon, with a sulphurous surface mottled in yellows and oranges. The gravitational forces of the massive planet Jupiter, less than half a million kilometres away, exert immense tidal stresses on the moon, which help to account for the dozens of active volcanoes on Io's surface.
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