Io was discovered by Galileo in 1610 while he was observing Jupiter. Io is a bright satellite that would be visible to the naked eye were it not for the brightness of Jupiter, very close by. Its size is very similar to that of the Moon, Mars or Mercury, and Io looks like a small planet.
The proximity of Jupiter causes very strong tidal effects which have caused very active volcanism by deforming its surface. Many volcanoes spew out sulphurous compounds on the surface of Io.
Various moons of Jupiter. From left to right: Io, Europa, Ganymede, Callisto