lo is the fourth largest moon in the solar system
It is the inner most of planet Jupiter.
With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io’s interior as it is pulled between Jupiter and the other Galilean satellites—Europa, Ganymede and Callisto. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (300 mi) above the surface. Io’s surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon’s silicate crust.
It was named after the mythological character of Io, a priestess of Hera [In Greek mythology]
Galileo discovered lo in 1610 Jan 7