Io is an inhospitable moon of Jupiter. Terraforming will prove extremely challenging but colonization of Io would be relatively easy using the geothermal energy to create an electromagnetic field so we could protect the colony from the radiation belt of Jupiter. This will allow people to move about on the surface without wearing protective suits. Due to the volatility of the moon, the volcanoes that have been dormant for centuries may start flowing without warning.
If we did put electromagnets in orbit space elevators would have to be made to transfer the geothermal power to the orbiting electromagenets. It is likely that Io will be colonized for geothermal power, it would be a great substitute for solar power this far out from the Sun. Io is also the only of Jupiter's four major moons with a solid rocky surface. Geothermal power and mined sulfur will be a large incentive, along with a wide variety of other materials.
Terraforming Io will be extremely challenging. It is extremely volatile and volcanic. It may be a good area for extremophile life to live.
Of note is that of all the moons in the Solar System, only Io has a surface gravity stronger than that of Earth's moon and has the most surface gravity of all the moons.
Io is a very active volcanic moon due to the gravitational pull from Jupiter. Another issue is that it receives lethal radiation from Jupiter's radiation belt. What would protect colonizers from both the radiation and volcanic activity? A world wide ocean, with maybe 5% land available for space and export infrastructure.
It would be easier to build the cities and farms that Io would need before the world was flooded. Power would come from geothermal, this power would be used to operate pumps and grow food. The power could also be used to manufacture items for the other Moons of Jupiter with cheap power. A secondary source of electricity could be generated from tidal forces of Jupiter pulling this ocean about. Io would have a very high energy output.
The water could come from two sources. 1: A rail gun on Europa firing 100 ton packets every few minutes (yes it would take a long time). 2: Import water rich comets from the Outer Solar System. Greenhouse gases would still have to be used to warm the Moon to create the ocean. Also due to the strong ionizing radiation, water will keep breaking down and will be lost to space, there are 2 possible solutions. 1: Let the water on Io freeze, but not in a deep freeze -20C maybe. 2: Keep the water levels topped up.
There is still an issue of natural disasters in Io, Volcanoes will have their flows slowed by the water, but they will not be stopped and will require monitoring and contingency to manage this issue.
Io can be given water, most likely from Europa or colliding comets. Enough so that a significant portion of the moon is covered in water. This will cause rains which will cool and stabilize the moon.
Life which thrives in sulfur can be added to the bottom of the ocean, plants and algae may be added to add oxygen. Cities may be built on the water or volcanically-inactive land to prevent destruction.
Plan DEditIn the far future, a possible option might be to launch the moon out of Jupiter's orbit and use it as a moon of Venus. Io is nearly the same size as Luna, just as Venus is nearly the same size as Earth. In a very distant future, ejecting moons and planets out of their orbits may be possible, if that's the case then Io would be a perfect candidate for a moon of Venus. This would allow us to terraform it somewhat more easily and more effectively than if it was a moon of Jupiter, because without the tidal forces the volcanos would cease spewing lava.
After cleaning out the sulphur (which can be stored for other uses), water would have to be introduced somehow and an atmosphere would have to be created. It wouldn't have to be very heavy and it wouldn't require many greenhouse gases. The only issue we might have to face would be the solar radiation, this can be remedied by using a plastic globe, effectively paraterraforming the new Io.