Io

Io is a volcanic Galilean moon of Jupiter, described in Horus Heresy sources as an orange world bearing Mechanicum habitats on its crust. The Galilean moons are the four largest satellites of Jupiter.

Heresy-era accounts record Io as a seething orange mass, its surface marked by continuous volcanic activity, with Mechanicum habitats established on the crust. During the Solar War, Io remained part of the Jovian industrial cloud, the complex of industrial holdings associated with Jupiter, and it survived that conflict as a component of it.

Io has no dedicated article on either of the fan-maintained reference wikis, Lexicanum and Fandom. The entry in the list of Jupiter’s moons is therefore the whole of the available record, and its standing rests on its volcanic character and its place within the Jovian industrial cloud.