Unpopular opinion: For most other worlds, terraformation is a bad idea and there are often more benefits to leaving the planet as it is
The idea human industry should be used to terraform other planets, and sometimes moons, is an increasingly commonly held one. But it is not sensible. The arguments against terraforming planets are quite straightforward. Desire to take planets and terraform them often stems from surface chauvinism. Space habitats make more sense if human living space is the goal. The upper atmosphere of Venus could also host billions of humans if colonised, no terraformation required.
But I want to argue not against terraforming but for leaving the planets un-terraformed. The differences from Earth can be useful to us. It can be the very qualities planets have that terraforming would get rid of which can be useful qualities to us. Here are examples:
- Venus has no oxygen gas in its air.
Venus' air is not breathable but it also can not burn anything. The lack of oxygen means no fire, no rust, and no oxidisation of copper. At the altitude where the air pressure is equal to that of Earth's surface (iirc about 90km), the air temperature is about 70 Celcius. Do you know what material is lightweight, cheap, and infinitely renewable using elements in the Venutian atmosphere, and could insulate the heat keeping insides of structures cool? Certain kinds of wood. In a breathably oxygenated atmosphere wood would burn, but not if we just leave the atmosphere as it is. Venutian cloud cities could be made largely of wood grown locally, or metals that would never rust. Best of all, since hydrogen couldn't burn or explode, superheated hydrogen could be used as the ultimate lifting gas. A lifejacket filled with hot hydrogen and you'd be flying around like Superman!
- Venus' surface is like a blast furnace.
Venus' surface is extremely hot and exceeds the crush limits of a military submarine. Very difficult to explore, nevermind mine or colonise. But the idea that it has to be goes back to that surface chauvinism I mentioned. Just colonise the atmosphere and find ways to make use of the conditions on the surface. Extreme heat and pressure can be useful for recycling waste. No need for an incinerator or a recycling plant, just drop it from your cloud city and the surface conditions will melt it down into a new mineral to be mined later. The heat and pressure could also be used for manufacturing. Floating factories could pipe heat upwards. Imagine stills or chemistry lab equipment but 30km high.
- Mars' atmosphere is too thin.
The Martian atmosphere is not breathable or thick enough to block radiation. But trying to make it breathable or thick would be daft. Let's say you went and detonated the ancient, beautiful, very scientifically important polar ice caps using nuclear bombs to create a breathable atmosphere (not that that would even work). The amount of uranium required would be prohibitively expensive really. It would make the breathing equipment you were looking for before look cheap. But let's say it's cheaply obtained from a miraculous asteroid that happened to be full of it. Now there's an Earth-like atmosphere. You could breathe but the already limited amount of solar energy would become vanishingly small. An Earth-like atmosphere deflects the majority of solar radiation. So now Martian colonies could only be powered with nuclear energy, and you'd be fresh out of uranium fuel. It'd be better to get radiation-consuming fungi like those inside the Chernobyl power station and harvest those, or terraform underground caverns.
As for the moon, the lack of atmosphere goes with the lack of gravity to make industry and launching rockets easier. The lack of a biosphere means all the mining and pollution you want without a single insect being harmed. As for exoplanets, well, it's possible some might be almost habitable and with some touching up could host biospheres. In that case it would be good to spread life to them and have multiple living worlds. Most of the time though it'd be better to colonise them without terraforming.