Out of all of the moons that orbit Jupiter, Io might be the least habitable! The surface of Io is covered in volcanoes that are completely active, and can project lava for miles and miles. As well as this, whilst Io has an atmosphere, most of it is made up of sulfur, with no oxygen for humans to breathe. Plus, the 3 other Galilean moons have water beneath their surface - we don't think this is the case for fiery Io.