Jan 7, 2010 – Volcanic plume above Jupiter’s moon Io – Michael Benson/Corbis

Io is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter  of 3,642 kilometers, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus.

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 by Jupiter’s varying pull. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (310 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. Some of these peaks are taller than Earth’s Mount Everest. Unlike most satellites in the outer Solar System, which are most composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Most of Io’s surface is characterized by extensive plains coated with sulfur and sulfur dioxide frost.