Multiple realities
(covers information from several alternate timelines)

Io was a moon of Jupiter.

In 2259, the secret Starfleet Section 31 construction hangar where the Dreadnought-class USS Vengeance was constructed was located in orbit of Io at coordinates (Star Trek Into Darkness)

On her way to intercept V'ger in the mid-2270s, the USS Enterprise flew by Io while passing by Jupiter and its moons at impulse. (Star Trek: The Motion Picture)

In VOY: "Life Line", Lewis Zimmerman sarcastically recommends a tour of the lava flows of Jupiter's third moon, which, accoring to real-world science, is Amalthea, a small asteroid. If Zimmerman's remark was intended as a reference to Jupiter's volcanically active fifth moon Io, the order of Jovian satellites must have changed by the 24th century.
According to the Star Fleet Medical Reference Manual, (p. 17) Humans discover dilithium in 2049 on the "fifth moon of Jupiter", which would be Io by today's numbering.

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