After the 32nd century, Io was terraformed, and became the headquarters of the Galactic Federation during its reign. (PROSE: Legacy) Lord Izlyr planned on taking Princess Lixgaar of New Mars to Federation headquarters on Io until New Mars was safe enough for her to return. (AUDIO: The Prisoner of Peladon)
Behind the scenes
In the real world, it is not the innermost moon, as more inner moons have been discovered.