Discovered by Galileo in 1610, when he pointed a telescope at Jupiter, Io is a bright satellite that could be seen with the naked eye if Jupiter does not dazzle us. With a size similar to that of the Moon, Mars or Mercury, Io has the appearance of a small planet.
The proximity of Jupiter causes effects of strong tides, deforming the surface of the body, making a very active volcanism. Multiple volcanoes are spewing sulfur compounds on the surface of Io.
Click here for an animation watching the rotation of Io. Credit : NASA.