Io is the innermost of the four Galilean Moons of Jupiter; that is to say the moons discovered by Galileo Galilei in January 1610. It is also the most volcanically active body in the Solar System, with an estimated resurfacing rate of 10 mm per year, due to the tidal influences on the small moon by Jupiter and the larger moons Europa and Ganymede; there are no visible impact craters on the surface of Io.
The Io has been studied in detail by six NASA spacecraft, Voyager 1, Voyager 2, Hubble Space Telescope, Galileo, Cassini, and New Horizons, giving scientists a reasonable picture of the surface. In 2006 the United States Geological Survey released a set of four global mosaics of Io. These have now been updated, and combined with new data, to create a downloadable Geological Map of Io.
Detail of part of the new Map of Io. United States Geological Survey.