Io is a moon of which planet?

Io is a moon of Jupiter. It is the innermost of Jupiter’s four large Galilean moons, discovered by Galileo Galilei in January 1610. The moon’s name comes from Io, a priestess in Greek mythology who was associated with Zeus, the Greek counterpart of Jupiter.

Io is famous for being the most volcanically active world in the Solar System, with more than 400 active volcanoes. Its surface is coated in colorful sulfur compounds, lava flows, and volcanic deposits rather than the thick water ice found on many outer-planet moons.

This extreme activity is caused by tidal heating. Jupiter’s powerful gravity continually stretches Io, while gravitational interactions with Europa and Ganymede keep its orbit slightly elliptical. Friction generated inside Io turns that orbital energy into heat, driving volcanism.

A common mix-up is confusing Io with Europa, Ganymede, or Callisto. All four are Galilean moons of Jupiter, but Io is the innermost and most volcanic. Io also rotates synchronously, meaning the same side nearly always faces Jupiter.