Discovered on 8 January 1610 by Galileo Galilei, Io is the innermost of the four Galilean moons of the planet Jupiter. Io is named after the priestess of Hera of Greek mythology who became a lover of Zeus.
At about 4.5 billion years old, Io is the same age as Jupiter. Larger than Earth’s Moon, Io is the third largest of Jupiter’s moons with a diameter of 2,263 miles.
Io is the most geologically active object in the Solar System with over 400 active volcanoes. As Io travels in its slightly elliptical orbit, Jupiter’s immense gravity causes “tides” in the solid surface that rise 100 m (300 feet) high on Io, generating enough heat for volcanic activity. Io’s volcanoes are driven by hot silicate magma. Volcanic plumes rise almost 190 miles (300 km) above the surface.
Io’s surface is dotted with more than 100 mountains, some of which are taller than Mount Everest. The volcano Loki on Io is the largest active volcano in the solar system. This single volcano may release more material lava than all of Earth’s volcanoes put together.
Except for the locations of the active volcanoes, the surface of Io is extremely smooth, composed of extensive plains coated with sulfur and sulfur dioxide frost. The volcanic action is so continuous that flow from the volcanoes smooths out any irregularities and erases impact craters. A thin atmosphere of sulfur dioxide exists on the moon, presumably from the eruptions.
Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io but not so hot to do so farther out.
Temperatures on Io vary from extremely hot to extremely cold. Areas which feature volcanic activity can reach temperatures of over 2000C (4000F). Away from these areas however will see temperatures plunge to -143C (-230F). Io is often referred to as a celestial body of fire and ice.