Far in the future technology has evolved to the point that humanity is able to try and terraform something in our solar system.

The moon is out, since it's too close to earth and therefore to risky, since we don't want any debris raining down on our home (assuming we use techniques like dropping comets and such). Also, all governments are already invested with industry on it and don't want that destroyed.

Mars is out, too, because we already established colonies of domes there and no one wants these destroyed / or unusable for a time either.

Which other place in our solar system is most suited for terraforming?

There are a lot of Saturn's and Jupiter's moons mentioned in the general listing of possible places, or Venus, but I have not been able to find an answer of the "most fit" or "least work to do".

If you want an Earthlike planet, and Mars is out, Venus is the only option. No other rocky body has gravity high enough to hold an atmosphere, especially close to the Sun where the solar wind is strong. (Titan is far enough away to not make this an issue, but it is VERY cold). It won't be easy, though.

It would require removing most of the atmosphere, which has a mass 93 times that of Earth's and consists mostly of carbon dioxide (96.5%) with the remainder being nitrogen. You would want to keep the nitrogen and remove all the carbon dioxide, but this sort of filtering might well be practically impossible. After that, you would have to add oxygen, though it is possible that the introduction of algae could turn some carbon dioxide into oxygen.

Venus is also way outside the habitable "Goldilocks" zone (too close to the sun), so it would either have to be moved quite a bit farther out (a huge endeavour) or be shielded from a lot of the sunlight it receives, e.g. with orbiting reflectors (which could double as energy collectors).

Venus also rotates very slowly, making for very long days and nights (there are two days to a Venus year) with extreme temperature differences between day and night. I suppose some of the solar reflectors mentioned above could direct some sunlight to the night side, but this would go against the purpose of limiting infall of sunlight.

It might be feasible with sufficiently advanced technology to direct a large number of comets towards Venus, where they would impact off-centre, increasing the planet's rotation while adding water to the atmosphere and eventually the surface. Hopefully, these impacts could strip off some atmosphere as well, but you would probably also need to kind of atmosphere cannon that compresses the carbon dioxide and shoots it off the planet. A problem may be that, given the mass of Venus, more comets would be needed than exist in the solar system without going out to the Oort Cloud (which is very far out).

Then there is the problem of Venus' very weak magnetic field, which provides negligible protection against cosmic radiation and particle radiation from the sun. Even if you provided Venus with an earthlike atmosphere, it would not be safe to stay outside for very long.

Given all these issues, I don't think it would ever be worthwhile to terraform Venus (or any other body in the Solar System, including Mars) when it is so much easier to create artificial rotating habitats built from asteroids or Kuiper Belt object. There's plenty of water ice out there from which oxygen can be created, and the nitrogen in our atmosphere could be replaced with helium, the second-most common element in the universe. The shells of these habitats would protect the inside from radiation and micro-meteorites, and you could have whatever climate you want inside. Given the amount of material to built such habitats from, you could easily house trillions of people, compared to a measly tens of billions on a terraformed Venus.

I know you said "But mars is out" But it really isn't.

Mars is out, too, because we already established colonies of domes there and no one wants these destroyed / or unusable for a time either.

You probably didn't establish colonies of Domes, because if you had the materials to withstand micrometeorites and the radiation issues from being on the surface of mars, then you've basically either already solved the terraforming issue on mars, or you've got the technology to terraform most arbitrary bodies in our solar system.

Just building domes in an un-terraformed mars is far more likely to result in destroyed or unusable for a time because of collisions not burnt up in a thick-ish atmosphere and radiation frying hardware, people, and structures.

What you would have built were underground structures, full stop. No radiation issues, no meteorite issues, and you don't even have to terraform. This also presents little issue to terraforming.

Solar wind has the effect of exciting air molecules enough to escape the gravity of a planet, and presents a radiation risk, but it appears that there are reasonable solutions to this problem ie not outside of current tech.

During the Planetary Science Vision 2050 Workshop[23] in late February 2017, NASA scientist Jim Green proposed a concept of placing a magnetic dipole field between the planet and the Sun to protect it from high-energy solar particles. It would be located at the L1 orbit at about 320 R♂. The field would need to be "Earth comparable" and sustain 50000 nT as measured at 1 Earth-radius. The paper abstract cites that this could be achieved by a magnet with a strength of 1–2 teslas (10,000–20,000 gauss).[65] If constructed, the shield may allow the planet to restore its atmosphere. Simulations indicate that within years, the planet would be able to achieve half the atmospheric pressure of Earth.

The "half of earths pressure" idea may or may not be realistic, but there are other ways to deal with this that I'll get to.

Then with that comes the issue of the fact that mars's gravity is simply much weaker than earths, and particles can escape the atmosphere on their own. Combine that with the fact that mars is much colder on average compared to earth, it appears that this will be accelerated when trying to heat up the planet with the terraforming process.

Source

I asked a question about this a while back on space exploration, and the conclusion I got was interesting:

it appears that water loss reasoning is in contention, and that the primary method of loss may have been through the weaker gravity, and not solar wind at all.

The loss of these particles happened over hundreds of millions to billions of years.

At the same time that same magnetic energy release powered a much stronger Solar Wind. The protons and other ions of the Solar Wind cause all the non-Jeans Escape processes listed in the Table above. Collectively several metres of water and perhaps 80 millibars of Carbon Dioxide would be lost over 4.2 billion years – at current rates of loss. As the bare minimum for terraforming is about ~300 millibars of carbon dioxide (equivalent to about 250 millibars of Oxygen) this doesn’t seem like a show stopper for terraforming. If we can supply modern day Mars with ~300 millibars in a few hundred years, then replacing 80 millibars in 4 billion doesn’t seem excessive.

If we were to provide mars with atmosphere, it might go away in 500 million years, but is that really that big of a deal on a human timescale?

So solar wind is not a problem on mars. Neither is losing atmosphere we get on mars. So what are the issues left?

• Atmospheric pressure


• Inert gas composition


• Sunlight


• Temperature


• Plantlife

Atmospheric pressure

With out proper atmospheric pressure, water, and you, will boil when exposed to the martian atmosphere directly. Liquid water will just boil off. If the lagrange point solar wind protector doesn't actually build up the atmosphere to half of earths, then here are you're options:

• you'll need to manually use mars's own materials to do so, this is probably possible in human timescales but there may not be enough atmosphere.




• you'll need to crash meteorites into mars to release enough gasses. Your colonies should probably be fine, and if you already have meteorite mining tech, you shouldn't have too much trouble making this happen. It will just take a long time to take asteroids off course to make them land on mars (100 years), and cut them up into enough smaller pieces that they don't accidentally blast more molecules than they insert onto mars, and don't put giant holes in mars itself. This is also farther outside our current tech.




• you'll need to transport gases from other planets instead of meteorites, this could take a bit longer, depending on how feasible it is to capture these gases and move them to mars.

You can also do any of these solutions part way, and then dig a deep hole in the planet, where air pressure is large, making open air environment at least for the hole you made in the ground. If you, say, could only manage to make mars's atmosphere 1/8 of air pressure at normal breathable points (ie 1/8 * 500 millibars, not the full 1000 at sea level, aka 62.5 millibars), you could actually just dig a hole so that you had 8 x the amount of air above you that you would have at sea level. That would be a really deep hole, but with much less volcanic activity, it is possible you could dig many times deeper on mars than you could on earth with out heat issues, and with less gravity, rocks may be easier to get through at a certain level. You could then wait for a full terraform if need be.

Inert gas composition

These gases would also need to have a lot of nitrogen, as the rest of the gasses that might work have adverse side effects, will escape the atmosphere easy or are rarer. With out such gases, Co2 will poison us in the concentrations required to make mars with an earth like atmosphere, similar story for O2.

Sunlight/Plantlife

The sunlight mars receives is significantly lower than earths, 44% of earths per unit area. However, A: many plants actually don't do well in direct sunlight (both in water and out) and prefer lower light. B: plants are primarily Co2 limited in many systems, not light limited (and will avoid trying to get all wavelengths of light because too much energy will destroy their chloroplasts and surrounding cells). So from just the sunlight perspective, it isn't an issue for many earth plants, though they may be focused at the equator and smaller at the start.

Temperature

The bigger issue here is heat (at least for plant life). Mars is significantly colder (though maybe not as cold as you would think):

Differing in situ values have been reported for the average
temperature on Mars,[22] with a common value being −63 °C (210 K; −81
°F).[23][24] Surface temperatures may reach a high of about 20 °C (293
K; 68 °F) at noon, at the equator, and a low of about −153 °C (120 K;
−243 °F) at the poles

About the only real way to really heat up mars is to thicken its atmosphere to get a greenhouse effect. One thing to note, 95 % of mars's atmosphere is Co2, while 0.0407% of earths is Co2. Average surface pressure on mars is 610 Pa, but pressure is affected by many factors including temp, so this is highly inaccurate, but if you compared this directly with earths 101325 Pa pressure, if we increase the pressure on mars by the increase in gravity to reach earth gravity, (1/.38 = 2.63...) we get 1605 * 95% to get 1525 pa, and then we get 1525/101325 = 0.01505% ... a very rough approximation of the comparison of the CO2 per unit area of mars if compared to the atmosphere of earth. This is less than half the amount of CO2 on earth. We would probably need to extract more CO2 from mars and get other greenhouse gases like methane to increase the greenhouse effect.

Plantlife

Okay, assuming we solved all the other issues here, we would still have a few more hurdles:

Plants still need oxygen.

Plants need good soil/medium to grow in.

The first part might be surprising, but you can actually suffocate many terrestrial plants in water if you completely submerge their roots, and especially so if you submerge the whole thing. To solve this you would need to bring Cyanobacteria to mars (blue green algea). This isn't a big deal (cyanobacteria grow fast) except you would need to have lots of water surface area for them to grow. You would then need to create an equilibrium with plants quickly after. You would probably need to grow both in isolation before releasing to make sure you hit a good equilibrium (not suck all the co2 out of the air and leave none for the plants). You would then also need other kinds of co2 producing organisms to take oxygen back into co2, probably more than just people. We'll get to this in a bit.

Soil is another story. You can't just use martian soil because it has virtually nothing that plants can use and is toxic to most plants and living organisms. Now, if you completed the previous steps, you might have lakes/oceans which might be big enough to cover lots of the planet with 35 meter oceans. This is fine, we don't need the Mariana trench for this to work. Instead of putting plants in soil, you can just use water plants/algea for the time being, where the Cyrano bacteria are sitting. But in addition to that, we'll need organic material to fertilize the plants. This comes from both the cyanobacteria and inveterate aquatic organisms that can feed on algea, like daphnea, but maybe even including snails, shrimp, etc... These can grow and exist in extremely limited environments already. These animals are already used to living in environments that emulate "low G" (water) and require very little to live. A small 20 once cup with algae is enough to sustain these creatures (though with proper atmosphere). What will happen is the waste will be used the fertilize plants, and will gradually build up on shores, where other types of plants will eventually be able to take root, and a slow process of plants + other animals being introduced will allow finishing the terraforming process.

The hardest part for us right now is Air pressure, if we can't find proper gas sources for this on mars. Otherwise we might actually have the technology, or close to it, today to complete this process in a couple hundred years.

The most suitable, and underrated planet is Earth!

And that's not just a joke answer. Even building habitations in Antarctica is 1000x easier to permanently settle than spending Trillions of dollars to get a manned base on Mars (or any other Solar System planet).

Antarctica has breathable air. It has water. It has protection from cosmic radiation. If something goes wrong, it can be resupplied.

Currently, 95% of the worlds population sits on just 10% of the land area. There are vast tracts of land still unsettled across the world. Even building floating cities on the Oceans would be far far easier than terraforming the other planets.

Full size link: https://en.wikipedia.org/wiki/World_population#/media/File:World_population_density_1994.png

If you are terraforming your first planet, you are probably K-type 1 to 2 civilization -- see https://en.wikipedia.org/wiki/Kardashev_scale

So, Titan. Heat isn't a problem. Cooling stuff off is often harder than warming things up. Gravity is hard.

Spinning up Venus, for example, is going to cost in the neighborhood of 10^29 J, or 10^12 seconds of a K1 civilization's power budget: a 30,000 year project.

Serious terraforming takes serious energy. And serious energy makes being cold a trivial problem.

The two hardest things are (a) getting rid of waste heat, and (b) gravity at a K1.5 civilization level.

Note that fuzing multiple planets into a larger one is going to result in a lot of debris and a lot of waste heat, and as mentioned, heat is annoying to get rid of.