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Io (eye'-oe, , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter and the fourth largest moon in Solar System. Unlike most satellites in the outer Solar System, Io is primarily composed of silicate rock surrounding a molten iron (or iron-sulfide) core. Thanks in part to this composition and tidal heating, Io has one of the most geologically active surfaces in the solar system, with over 400 active volcanoes and more than 100 mountains, some taller than Earth's Mount Everest.[1]<sup>,</sup>[2] Several of Io's volcanoes also produce volcanic plumes up to 500 km (310 mi) high, composed of sulfur and sulfur dioxide. These in turn produce large surface changes and provide material for Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere.