innermost of the 4 Galilean moons of Jupiter
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Io is the innermost of Jupiter's four largest moons, discovered by Galileo in the early 1600s. It is scientifically significant because it is the most volcanically active body in our solar system, with hundreds of active volcanoes driven by intense internal heating.
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Apparent magnitude 5.02 (opposition) Angular diameter 1.2 arcseconds Atmosphere Surface pressure 0.5 to 4 mPa (4.93×10 to 3.95×10 atm) Composition by volume90% sulfur dioxide
Io (/ˈaɪ.oʊ/) is the innermost and second-smallest of the four Galilean moons of Jupiter. Slightly larger than Earth's Moon, Io is the fourth-largest natural satellite in the Solar System, has the highest density and strongest surface gravity of any natural satellite, and the lowest amount of water by atomic ratio of any known astronomical object in the Solar System.