Io is the innermost large moon of Jupiter. It is the closest of Jupiter’s four Galilean moons and orbits between Jupiter’s smaller inner satellites and Europa.
Galileo Galilei first observed Io in January 1610, along with Europa, Ganymede, and Callisto. Their discovery provided the first clear evidence that bodies could orbit a planet other than Earth, challenging the Earth-centered model of the Solar System.
Io is famous for being the most volcanically active world known. Jupiter’s immense gravity, combined with repeated gravitational tugs from Europa and Ganymede, flexes Io’s interior and produces intense tidal heating. More than 400 active volcanoes have been identified, with plumes of sulfur and sulfur dioxide rising hundreds of kilometers.
A common mix-up is the order of Jupiter’s large moons by size: Ganymede is the largest, followed by Callisto, Io, and Europa. But by distance from Jupiter, Io comes first among the Galilean moons.