Io (
/ˈaɪ.oʊ/), or
Jupiter I, is the innermost and third-largest of the four
Galilean moons of the planet
Jupiter. Slightly larger than
Earth's
moon, Io is the
fourth-largest moon in the
Solar System, has the highest density of any moon, the strongest surface gravity of any moon, and the lowest amount of water (by
atomic ratio) of any known
astronomical object in the Solar System. It was discovered in 1610 by
Galileo Galilei and was named after the mythological character
Io, a priestess of
Hera who became one of
Zeus's lovers.With over 400 active
volcanoes, Io is the most geologically active object in the Solar System.=10.5px 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 moons—
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 Io's
silicate crust. Some of these peaks are taller than
Mount Everest, the highest point on Earth's surface.=10.5px Unlike most moons in the outer Solar System, which are mostly 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 composed of extensive plains with a frosty coating of
sulfur and
sulfur dioxide.Io's volcanism is responsible for many of its unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various subtle shades of yellow, red, white, black, and green, largely due to
allotropes and compounds of sulfur. Numerous extensive lava flows, several more than 500 km (300 mi) in length, also mark the surface. The materials produced by this volcanism make up Io's thin, patchy atmosphere and Jupiter's extensive
magnetosphere. Io's volcanic ejecta also produced a large
plasma torus around Jupiter.
Source: Wikipedia 
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