|Io as seen from Galileo.|
|Diameter|| 3,642 km|
|Distance from primary|| 421,600 km|
Io is the innermost of Jupiter's four planet-sized moons. Lava flows and irregularly shaped mountains are also observed, but no craters (consistent with ongoing volcanic activity, which would erase impact craters not long after they form). The composition of these lavas and plumes remains uncertain. Io is the same size and density as Earth's Moon.
Jupiter's strange moon is literally bursting with volcanoes. Dozens of active vents pepper the landscape with volcanic rings the size of California. The volcanoes themselves are the hottest spots in the solar system with temperatures exceeding 1800 K (1527 C) about 1/3 the temperature of the surface of the Sun. The plumes which rise as much as 500 kilometers into space. Confounding common sense, these high-rising ejecta seem to be made up of, not blisteringly hot lava, but frozen sulfur dioxide. For a world dominated by fiery volcanoes, it's curious that Io is also very, very cold. The ground just around the volcanic vents is literally sizzling, but most of Io's surface is 150 degrees or more below 0 C.