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Terraforming Gas Giants
A gas giant is a giant planet made almost completely of gasses. The Solar System hosts four gas giants. Around other stars, other similar planets exist. For a classification of gas giants, see Gas giants (theoretical models).
In theory, there are many ways a gas giant can become useful for settlers. Some methods don't actually try to make these planets habitable for any living organism, while other methods propose ways to transform them into at least partially habitable worlds.
Non-Terraforming Methods[]
Artificial Sun[]
The idea is that nuclear fusion reactions can be started inside the core of a gas giant. Giant planets have enough hydrogen, but their mass is lower then what is needed to start a thermonuclear reaction on its own. If they had enough mass, they would have already become M - type stars or at least Brown Dwarfs. Still, there is the idea, at least in science-fiction movies, that large enough giants (like Jupiter) can be artificially transformed into new stars.
If hydrogen or at least deuterium fusion was somehow started inside the core of Jupiter, the planet would expand, and internal pressure would decrease until it no longer matches the conditions required for nuclear reactions to continue. The artificial star would be short-lived. Still, the time needed for the new failed star to cool down is longer than a human lifetime.
Another way to heat-up a gas giant is to create a False Star. All we need is just to heat-up the planet until it reaches 3000 degrees, which is enough to create the illusion of a star. It would then take centuries to cool down. This could be done with huge explosions. However, theoretical models suggest that many gas giants have hot layers of gas below their upper atmosphere. If somehow we managed to mix-up their gas layers, we could bring heat to the upper layer. This process would need to be maintained over centuries, in order to keep temperatures high enough.
The Impact Theory could also be applied to gas giants. If we managed to collide two large objects (e.g. a Jupiter-class planet with a Neptune-sized one), the impact would generate the necessary heat and also mix their atmospheres together. However, the energy needed to disrupt the orbits of two giants is huge and only a highly-advanced civilization would be able to do so. Also, there would be a high risk of collision between the diverted planet and the moons. Increasing the mass of the main planet would also affect orbit of its moons. This method is too risky to use.
Another method to heat-up a gas giant is the Moon Impact Theory. This would not work for the gas giants in our solar system, as they do not have large enough moons. Still, there is a possibility that some gas giants in other systems might be binary. Imagine a Jupiter with a Neptune-size moon orbiting closely (like Pluto and Charon), and other moons in more distant orbits, some of them large enough for human colonization and orbiting the binary. Colliding the planet and its giant moon would have much effect on the motions of outer planets, but it would heat the planet by up to 2000 degrees.
Artificial Luminosity[]
In one sci-fi novel, a self-replicating lantern was thrown into Neptune. After many years, it managed to create many copies of itself, floating in the upper part of the atmosphere and creating enough light to sustain plant life on Triton, which was terraformed with the help of greenhouse gasses.
The idea might sound impossible with current technology, but it might be possible to create a fleet of nuclear-powered stations, each floating like a hot-air balloon, in the upper atmosphere of a gas giant, providing the light needed for human colonies on the gas giant's moons. The process could also generate a limited amount of heat.
Industrial Usage[]
Gas giants are large and partially recycle their atmosphere. Extracting raw materials from them would be almost impossible, and in any case mining asteroids is much easier. Still, to avoid filling space with debris and covering planets and asteroids with garbage, many would agree that dumping things into a gas giant is a much better alternative.
Possible Colonization Methods[]
Creating a colony on a gas giant might sound impossible. If you tried to land on a giant planet, you would only descend into hotter and denser gasses, until the pressure caused your ship to implode. They don't have a solid surface. Also, their winds are a real problem, reaching over 1000 km/h. They have huge storms, hurricanes that can be larger then the diameter of Earth. Lightning has been noted to exist on Jupiter and Saturn. The change in temperature between gas layers also means that rain may occur between different layers of the atmosphere. It might also be possible for snow or hail, made of various materials, to occur.
So, is it possible to create a colony on a gas giant?
Balloon Colony[]
The easiest way would be to create a floating station. It would be a giant hot-air balloon, floating inside the upper layers of the atmosphere, where the pressure is acceptable for humans, the weather is not quite so nightmarish and the temperature can be within tolerable limits. The colony will be on a continuous movement around the planet.
A large colony is not possible in this way. Strong winds would soon tear it apart. We can imagine a set of several balloons connected with a cable, forming a row. Another version is to have a large balloon at high altitude and a counter-weight many kilometres below it, with the colony somewhere in between. Since lower air is hotter, temperature differences between the balloon and the counter-weight could be used to generate electricity.
The balloon colony has two major problems. First of all, gas giants have a large amount of hydrogen and helium in their upper atmosphere, which means we cannot use a lighter gas to keep the balloon up. We would need to heat it. The second problem is that creating a launch pad for spacecrafts on a floating town is very difficult. Ship platforms will always be moving and will not support too much weight. Escaping the gravity of a gas giant requires much more energy (and larger ships) than escaping the gravity of a rocky planet.
Large Floating Colony[]
One theoretical model suggests that winds on a gas giant would not be felt at their real speed. This is because any floating object would be moving with the same speed the gas is moving. If this is true, then a city could float above a gas giant. Also, it is possible that some giant planets are not as windy as others. We know that Uranus has less powerful winds then Neptune. In theory, as a gas giant gets older, it loses its internal heat, which powers its winds and hurricanes. So, a planet old enough (e.g. a gas giant orbiting Barnard's Star) could be suitable for the creation of a floating city.
Construction of a floating colony would be very difficult, because many materials will have to be imported from elsewhere. We cannot build with gases. That said, gas giants do contain traces of heavier elements such as carbon, which could be used for a colony. A critical factor will be the equilibrium of forces. Hot air balloons (or maybe hydrogen balloons if the atmosphere is heavier) must counter the weight of the whole city. If anything bad were to happen, the city would sink to the abyss or rotate with one side up.
Energy could be generated using wind turbines. There should be speed differences between various layers of gas.
Microbe Terraforming[]
It has been theorized that small organisms (like algae and bacteria) could survive on a gas giant. These planets might have all the ingredients needed for life. Microbes can survive in the currents, as long as the pressure does not increase too much. Some scientists argued that Alien life might already exist inside the clouds of some giants.
It might be possible for us to insert life forms from Earth to create a breathable atmosphere of a gas giant. This may be possible with the use of genetically-modified algae to produce oxygen, which humans and many other organisms needs to breathe. Dealing with the excess hydrogen and other gases would be tricky, but giving a gas giant a breathable atmosphere would make it easier to construct floating colonies.
Complete Terraforming Models[]
Is it possible to completely terraform a giant planet? In theory, it is. There are two methods that could do this:
Atmosphere Removal[]
To remove the atmosphere of a gas giant is very difficult, and outright impossible with our current technology. Also, sending such a huge amount of gas into outer space would have catastrophic consequences. The gas would interact with the solar wind, creating huge radiation belts and solar storms. As the gas is pushed away, it could affect colonies on planets and asteroids it encounters. For colonies on the local moons, it would be a nightmare.
After removing excess atmosphere, the mass of the giant planet would decrease with more than half. A significant part of its moons will escape into solar orbits. The freshly exposed core would be very hot, probably well over 10,000 degrees. So we would have to wait a long time for it to cool down, unless we have the technology to speed up the cooling process.
Encased Planet[]
Another theoretical model suggests the construction of a large artificial surface above the atmosphere, like a giant shell. The surface would need to be extremely strong to support the forces beneath it. No known material is suitable for this. Only an extremely advanced civilization might be able to create an encased planet.
The Universe is full of giant planets. They exist in the Solar System as well as around many nearby and distant stars. Giant planets have been discovered also to be free-floating around the galaxy. Even if we might never be able to terraform them, understanding the importance they have in each solar system and the complex processes that happen inside them and around them is vital if we want to colonize their moons.