I looked up the list of celestial objects within the Solar System (http://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size), but found out that surface gravity is insufficient for most of them. It means that they will have trouble keeping their atmospheres unless their air is heavy. I then looked up the periodic table to see if it would be possible to replace nitrogen with something else to create heavy breathable air. Well, there is no easy way out. The best hope is to increase the amount of oxygen in air composition, either as carbon dioxide or simply oxygen. Don’t worry. A higher amount of oxygen actually makes you very happy and alert. I actually wish we could increase the oxygen level in our atmosphere today. The downside of such a modification is, of course, the increase in the size of bugs. For example, a dragonfly can grow to the size of an eagle if there is sufficient oxygen in the atmosphere. However, on Mars where there is no insect yet, this is perfect. We can keep them in solarium-based farms and gardens. Anyway, the current composition of the Earth’s atmosphere is perfect. It’s not that we have many options.
Even Mars is too weak compared to other more interesting planets, such as Venus. Given that the Sun will not turn into a red giant any time soon, Venus is actually a far better bet than Mars. In fact, Venus is a true companion of the Earth if you compare the stats. A carbon-oxygen bond is very strong, making it difficult to modify the atmosphere of Venus using conventional chemistry. Once Venus has a brand new atmosphere, the temperature will automatically adjust to the correct level for human habitation. Do we need oceans? Well, if 1-2% of the surface of Venus is water, that’s more than enough for billions of people. The question is about where to get the missing hydrogen. Shipping from the Earth is not out of the question, but the ability to make hydrogen atoms from heavier elements without going through nuclear fission will be more interesting. It’s not easy, and even harder is to do so without releasing a destructive amount of energy. Recycling water is probably still the only way to go without a further understanding of quantum physics.
Many planets and moons have missing elements. How to make brand new atoms from other elements at a large scale is still the key technology to unlock the future of terraforming.
Actually, I have changed my mind. Jupiter and Saturn will be the ones to convert. Build spaceships and just float. Why do we need land anyway? Plus, it’s an ocean of fuel for you to go anywhere you want within the Solar System. In fact, we can even do this on the planet Earth, where submarines run on nuclear fusion by using sea water as its fuel to create mobile self-sustainable biospheres of their own. But, don’t we have to synthesize food on the ships? Nay, just eat seafood every day.
Submarines are invaluable for the military and economic development of a great nation that wishes to have a global presence. After all, 70% of Earth’s surface is water. You can go anywhere you want.