The Red Mars / Green Mars / Blue Mars trilogy by Kim Stanley Robinson describes the process of terraforming Mars so that humans could live there. Could those same techniques work on other planets in the solar system?
For example, could those techniques work on a gaseous planet such as Saturn? Or do they require solid ground? Similarly, could they work on a planet with a dense atmosphere such as Venus, or is that planet too dissimilar to Mars to work without major changes?
The same terraforming techniques used in the Red/Green/Blue Mars series wouldn't work on a gas giant. But some form of terraforming may. The thing about gas giants is that their atmospheres are layered and of varying composition. Their cores are so high pressure that they are essentially molten. We'd never be able to live down there. However, many gas giants probably have a layer in their atmosphere that would be the right pressure and temperature for human habitation. It may not have the right atmosphere composition.
Most gas giants have a primarily hydrogen/helium atmosphere. Mars' atmosphere is primarily CO2. So in the Red, Green, Blue Mars series, what they are doing is causing runaway global warming (releasing more stored CO2) to increase atmospheric pressure and surface temperature. Then they are planting plants to convert the CO2 to O2.
It's probably not possible to use similar atmospheric modification techniques to adjust the composition of gas giants. It depends heavily on the composition of the atmosphere at the correct temperature and pressure layer. There are trace amounts of CO2 and Oxygen. And since the layers stratify, it's possible that there could be an oxygen layer or a CO2 layer. They may or may not line up with the correct pressure and temp. If an oxygen layer does, we're in good shape -- no terraforming required. If it doesn't or a CO2 layer does, then its likely that it would not be modifiable, because any oxygen created would rise or fall to a different layer. If, by some strange coincidence the oxygen layer is only slightly above or below the CO2 layer, that is the one circumstance where similar techniques could work. Grow the O2 layer and it will take up the space (maybe) vacated by the CO2. On the other hand, if the correct layer is helium or hydrogen, then the techniques used on Mars would not work. Period. Also, this would require some sort of technology that would allow us to float platforms or airships pretty much indefinitely with out too much energy expenditure at the correct level in the atmosphere.
In Venus' case, the problem is the reverse of that on Mars. On Mars, the terraforming involves causing runaway global warming – basically. On Venus, the problem is reversing it. So it would require completely different techniques.
The main steps that are used, if I recall correctly, are (excluding things that were done just to survive):
The planets we have are (excluding Earth and Mars):
Mercury: Much hotter than Earth: rather than heating the atmosphere, as in Mars, the task would be cooling it (so the windmills, asteroid hit, and moholes would not be useful). There are no polar caps to melt. There's very little atmosphere, so rather than trying to change it, the task would be creating a suitable one - the Mars techniques (assuming all the other problems are solved) would probably be useful here.
Venus: the atmosphere is much more dense than Earth, and it has the same heat issue as Mercury. There is no magnetic field preventing depletion of the atmosphere (i.e. it's constantly replenished), so whatever changes made (e.g. the GEMs, lichen, algae) would be unable to build up, like they did in Mars (unless an earlier step somehow added a magnetic field).
Jupiter: it's made up of helium and hydrogen. Even if it could somehow be transformed into something Earth-like (but huge) the Mars techniques would be of no use. There are, however, 63 known satellites, and Ganymede, Callisto, Io, and Europa are similar in many ways to the inner planets.
Saturn: much the same as Jupiter (i.e. the Mars techniques are no use). 62 known satellites; Titan and Enceladus show signs of geological activity but are mostly made of ice.
Uranus: much the same as the other gas giants. Given the distance from the Sun, it seems likely that not only would the Mars techniques not work on the planet, they would be insufficient to heat the moons as well.
Neptune: like Uranus, but colder.
As @Pearsonartphoto suggested, the satellites (particularly Jupiter and Saturn's) are much more suitable targets, for example:
Phobos is destroyed in Red Mars, which also helps the terraforming (although that wasn't the goal) by adding heat. This wouldn't be possible on one of Jupiter/Saturn's moons, since it's not moons all the way down.
They couldn't even work on Mars -- Robinson accelerated the timescales by a few orders of magnitude for dramatic purposes.
The same techniques would not work on a planet, but they might work on a moon of the outer solar system. Of course, Titan is already covered in a greenhouse gas to a higher pressure than the Earth, and it's not particularly warm. It might help some, but it probably wouldn't be the end-all solution.
Gas Giants are really impossible to change the composition of. Firstly they are massive: tens to hundreds of times the mass of the earth, and mostly hydrogen. Any Oxygen created would combine with the hydrogen (and possibly other elements), and probably fall out as rain/snow, and be lost into the lower atmosphere. In fact the gases probably are convectively mixed, so you can't just concentrate on a thin layer with roughly the right temperature and pressure and a reasonable net mass, but you have to change the composition of the entire planet. If you had that much stuff available, you could construct one or several planets yourself easier than trying to change a gas giant.
I also think water worlds (with deep oceans) are a serious problem, nutrients would sink into the depths, which in a true water world might be hundreds of kilometers. Unless some sort of geologic activity keeps mixing it up (like plate tectonics does on the earth), any compounds that are preferentially deposited in sediments are rapdily (on a geologic timescale), lost to the potential biosphere.
If you had really really vast amounts of energy you could build artificial suns and cover pretty much any ball of rock with the gasses necessary to sustain life, at least for a while. Seems like a lot of hassle, though. It makes no sense to terraform an entire planet when you can make perfectly liveable city-sized domes at far less cost.
After all, if we wanted to live in an atmosphere, why would we bother leaving Earth in the first place?
I don't think they would work on Mars. First the timescale is hugely sped up. Mars society estimates a thousand years to get to a point where the air is thick enough for trees to grow and humans to be able to survive without pressure suits with an aqualung type closed system breathing apparatus. And it could be thousands, even hundred thousands of years, to get to an oxygen rich atmosphere.
Carbon dioxide is poisonous to humans above 1% concentration in the atmosphere, even with plenty of oxygen. And it depends on there being enough CO2 for the atmosphere to go into a runaway greenhouse effect.
It's not at all clear if there is enough. For a runaway effect you need 10% of Earth's atmospheric pressure. There is enough known for 2% of Earth's atmospheric pressure.
The amount of energy needed to liberate dry ice is huge. About a billion megatons of energy to double the atmospheric pressure or about nine billion to get to the 10% concentration where a runaway greenhouse could start up.
If you deliver that much energy to Mars, that's a rate of several megatons of energy you need to supply every second, all just going into making the dry ice sublimate - never mind warming it up, or the ice, or the regolith and ignoring all losses into space.
His techniques would generate several orders of magnitude too little energy to do much to the climate of Mars.
Which is no surprise. Look at Earth. It takes billions of people driving cars and burning coal for decades, to make a difference to the temperature of the Earth of one degree. And indeed the "easiest" way to warm up Mars is probably to create artificial greenhouse gases using fluorite ore. But that is still a mega project. It's eleven cubic kilometers of fluorite ore you need to mine, and it requires the output of 200 nuclear power stations running for a century, just making greenhouse gases as the only thing they do with all that power. And it only works if Mars has enough CO2. And there's lots to go wrong along the way.
Earth's atmosphere is not nearly warm enough for Mars even if you could duplicate it magically on Mars. If you manage to get rid of all the CO2 and replace by nitrogen and oxygen, take the carbon out of the atmosphere so it is breathable - then you are committed to stepped up levels of greenhouse gases for all the future to stop it from getting as cold as Antarctica.
For the energy requirements see my article: Why Nukes Can't Terraform Mars - Pack Less Punch Than A Comet Collision
For other issues see my article: Trouble With Terraforming Mars
I think a huge issue is the use of Earth-borne plants and organisms to terraform a planet to our specifications. First the effort to terraform even one celestial body is titanic in proportion; we breath at 21% Oxygen, but we could survive at 15. Then there's the factor of Air pressure (14.7 psi) and temperature. Increasing air pressure may not be viable as the gravity may be too low for an increase in air pressure. As for atmospherics, the amount of oxygen needed to change to shift the percentage would be astronomical. I assume one could start with lichen and bacteria, something with very low water needs, and perhaps a small meteor to hit one of Mars' polar ice caps to start a global warming/nuclear winter catastrophe. It would raise the temperature, melt the water and introduce rain clouds. That could increase air pressure (albeit at a low percentage) and then introduce Earth bacteria and lichen to help produce oxygen, which the mix of oxygen-exhalation and moisture would increase the percentage. But you're talking a massive volume. I couldn't even begin to guess how many billions of metric tons, or particles per square inch, we would be talking about. Such a venture would probably bankrupt every government on the planet.
Now 'culturing' a native life form via genetics might be easier, cheaper, and possibly more viable. There is bacteria on Mars. A little testing, some Earth-bound DNA injected, and we could possibly make a hybrid/mutant strain that starts the first step of life. Still would take a few epochs for viability, but it's a thought.
