Which Galilean moon is closest to Jupiter? Io is the closest of the four large Galilean moons.
Io was the innermost of the four satellites Galileo Galilei observed around Jupiter in 1610. Its orbit lies inside those of Europa, Ganymede, and Callisto, so Io completes a revolution in only about 1.77 Earth days. The moons are named after figures associated with Zeus in Greek mythology, while “Galilean moons” honors their first telescopic observer.
Io is not merely the nearest; it is also the most volcanically active world known. Jupiter’s powerful gravity, combined with repeated gravitational tugging from Europa and Ganymede, flexes Io’s interior and generates intense heat. That energy drives enormous volcanoes and sulfur-rich surface deposits.
A common mix-up is to confuse orbital distance with size. Ganymede is the largest Galilean moon, and Callisto is the outermost, but neither is closest to Jupiter. Europa is second-closest, followed by Ganymede and then Callisto.