One of the four Galilean moons of Jupiter number I, the nearest to the planet and arguably the most remarkable. Its surface is brightly coloured - much of it a greenish yellow dappled with patches of orange and white. Eight active eruptive centres were identified in the images returned during the encounter of the Voyager 1 spacecraft, six of which were still active when Voyager 2 flew by four months later. Continuous monitoring from ground-based observatories, and comparisons between Voyager and Galileo images confirm the high level of continuous eruptive activity on Io. The eruptive centres appear as dark spots. Many are surrounded by roughly circular haloes of ejected material, and lava flows are also visible. The coloured crust is made of sulphur and solid sulphur dioxide. No impact craters are seen; any that were formed in Io’s early history have long since been covered by erupted material. Io is the only body in the solar system, apart from the Earth, definitely observed to be volcanically active, though Triton and Enceladus show evidence of likely activity, and Venus might also be active. Io’s activity was predicted on the basis of the strong tidal effects Jupiter has on the interior of Io. The satellite is surrounded by a thin atmosphere of sulphur dioxide and a ring of electrically charged particles - a plasma torus surrounds Jupiter, enclosing the orbit of Io. Data from Galileo indicates that Io has a substantial metallic, electrically conducting core.