Ganymede is the largest moon of Jupiter, and also the largest moon in the whole Solar System. Like other large moons of Jupiter, Ganymede is an interesting target for human colonization. Colonization of Ganymede won’t be easy, but not impossible, either.
The diameter of Ganymede is about 5 270 kilometres, which is a bit more than the diameter of Mercury. The mass of Ganymede is about 2.5 percent of the mass of the Earth, and only about the half of mass of Mercury. Gravity on Ganymede is about 15 percent of the gravity experienced on Earth.
Ganymede is mostly composed of rocks and water. Ganymede has most likely an iron core, that is partially molten. On top of the core, Ganymede has a mantle that is composed mostly of ice. Ganymede probably has large subsurface ocean of water. The crust is composed of frozen water. Ganymede has a magnetic field, most likely generated by its core.
Orbit and rotation
Ganymede orbits around Jupiter. One orbit around Jupiter takes Ganymede about seven days. The orbit has been synchronised with the orbits of Europa and Io. When Ganymede orbits around Jupiter once, Europa orbits two times and Io four times around Jupiter. Like most other moons, Ganymede is tidally locked, to the same side always faces Jupiter. On the sky of Ganymede, Jupiter is almost a static object.
The orbit of Ganymede is quite close to Jupiter, so it is in the radiation zone caused by the magnetic field of Jupiter. The magnetic field of Ganymede protects it from this radiation, to a degree.
There are two common geographical formations on Ganymede. About a third of the moon is covered by old, heavily cratered ice. The ice also contains clay and other substances. Two thirds of the surface are covered by newer ice, which is also quite cratered.
According to current knowledge, Ganymede does not have an atmosphere. There are some signs of a weak atmosphere, that would consist of oxygen, for example, but there is not yet definitive measurements. In any case, the people on Ganymede would have to wear protective suits, unless the moon is terraformed.
Because of the large distance from the Sun, Ganymede is a cold place. The temperatures range from -200 °C to -120 °C, the average being around -160 °C. The temperatures can be higher under the surface, and for example, in the liquid ocean, the temperatures would be suitable for life.
Ganymede is located quite far, so traveling there with current technology takes years. If we would be to use the same technology that NASA has proposed for colonizing Callisto, a round-trip would take about four years. Its not unbearable for humans, and with the advancements in technology, travel time would probably be shortened.
Building permanent habitats on Ganymede would be challenging because of the low temperatures and large distance from Earth. Most likely same technologies that have been proposed for other moons of Jupiter could also be used to colonize Ganymede.
First bases could be built on surface, that would protect the colonizers from cold temperatures and other harsh conditions. After that, we could expand the base underground, and the temperatures on subsurface oceans would be more suitable for life. The ice would also provide protection from the radiation, even though Ganymede has a magnetic field that protects the colonizers already to a degree.
Because of the distance from the Sun, using solar power would be hard. The best option would be to use nuclear power. For fission reactors, the fuel could be brought from the Earth. If fusion power is developed, helium-3 to power the reactors could be brought from Jupiter.
Depending on the position of Jupiter and the Earth, it takes between half an hour to an hour for the messages to be delivered. This would make real-time communications impossible. At times, because of the location of the planets and the sun, communications would be cut. This could be circumvented by using satellites.
Not much is known about the available resources of Ganymede. The most important resource, water, is commonly available on Ganymede. Therefore, the habitats could be quite independent, once we have built them.
Ganymede, like other moons of Jupiter, could be used as a waypoint when researching the gas giants. Rocket fuel could be produced on Ganymede for this purpose. Because of the relatively small gravity, it’s easy to leave and Jupiter could be used to speed up the spacecraft.
There are some theories on about how should we terraform Ganymede. Ganymede has similar problems, like the other moons of Jupiter. The important resource for terraforming, water, is readily available.
To terraform Ganymede, we would need to create an atmosphere for people to move freely without protective suits, and also to cause a greenhouse effect that would warm up the moon. Strong greenhouse gases could be brought to Ganymede to increase the temperatures. The atmosphere should be very thick in order for the temperatures to rise.
Oxygen, that is required for breathing, could be created from water by electrolysis. The process takes a lot of time and energy, though. Also, we should figure out where to deposit the extra hydrogen also created by electrolysis. Hydrogen could be transported to Venus to terraform it.
If we were able to warm up the planet and create an atmosphere, the large amount of water could pose an additional problem. If the water is plenty, as it is thought, the whole moon would be covered by a single ocean. We should then remove the excess water, or build artificial continents and islands.
Ganymede is the largest moon in our Solar System. It has a lot in common with other large moons of Jupiter. It already has a magnetosphere and large amounts of water. Because the temperatures are cold and it lacks an atmosphere, colonizing it would be challenging.
Terraforming Ganymede could be possible in the future, but this takes a lot of resources and time. We can only dream about the terraforming of Ganymede, but it might happen some point in the future.