Io is the innermost of the four Galilean moons of the planet Jupiter. Discovered in 1610 by Galileo Galilei, Io is the fourth-largest moon in the Solar System, has the highest density of all moons and has over 400 active volcanoes. With a diameter of 3642 kilometers, Io is slightly larger than Earth’s Moon. Io is unusual because it is actively volcanic and has a thin atmosphere composed primarily of sulfur dioxide.
Io was named after one of Zeus’ lovers in Greek mythology. Its volcanic activity was discovered in 1979 by Voyager 1 during its flyby of Jupiter. Since then, several other spacecraft have visited Io and have photographed extensive surface changes resulting from frequent eruptions. Io currently has dozens of active volcanoes, with Stromboli being one of the most active volcanoes on Io today.