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In terms of practicality, which of the planets proposed for terraforming would require the least resources to make fully habitable? In other words, which would be the most practical given technology available today or in the foreseeable future?

Would it be somewhere like Mars, which is already relatively hospitable but large, or something smaller but more alien, like Ceres?

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    $\begingroup$ I don't see anyone mentioning the Earth...? :) $\endgroup$ – a CVn Jun 6 '14 at 18:27
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    $\begingroup$ The answer really depends on what you mean by terraforming. Is a domed building sufficient? How about a hollowed out asteroid? What about a planet covered in a dome? What about a planet where the valleys are habitable, but not the mountains? Or is the only thing acceptable a planet that you can walk anywhere on, like Earth? $\endgroup$ – PearsonArtPhoto Sep 14 '16 at 14:38
  • $\begingroup$ Gwenn, per Pearson's comment, please clarify your question. $\endgroup$ – called2voyage Sep 14 '16 at 14:53
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Here are a list of properties I consider to be needed to make a planet Earth-like:

  • Solid or liquid surface at an atmospheric pressure of about 1 bar.
  • Atmospheric constituents suitable for plants, animals, fungi, bacteria and so on to survive unaided. This basically means copying Earth's ratios as close as possible.
  • Tectonic activity or technological equivalent, to replenish the atmosphere.
  • Gravity similar to Earth's.

The hardest thing to change about a planet in terms of bringing it close to Earth in all properties that matter to Humans, would be its gravity.

Whilst the atmosphere of Mars is closer to Earth's at the present time, it will always be one fifth of the mass of Earth, and will therefore never (unless we add mass to it) be close to Earth's gravity.

Therefore, I am of the opinion that Venus would be a better terraforming target. Thinning out its atmosphere, e.g. by dispersing bacteria into the air, which will turn gasses into solids or liquids to then rain down to the surface, or by putting a big Mylar parasol in front of the planet to cool it down and freeze-out some molecules, would be comparatively much easier then smashing hundreds of moons into Mars to make it more massive.

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    $\begingroup$ The definition you use of terraforming is really limiting to your case here--Rody's definition is a far more standard understanding of terraforming. We don't need to make a planet exactly like Earth, but instead just enough like Earth for it to be easily habitable and long-term sustainable. $\endgroup$ – Gwen Jul 17 '13 at 15:24
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    $\begingroup$ @Gwenn Oh well. I don't like roller-coasters, bungie jumping or trampolines either. Me <3 terra firma @ 9.8m/s². $\endgroup$ – Nicholas Shanks Jul 17 '13 at 15:54
  • $\begingroup$ There was something about the Venusian clouds being strongly acidic in nature; puts a crimp in the bacteria. $\endgroup$ – Everyone Sep 4 '13 at 18:23
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    $\begingroup$ @Everyone Acid is no problem at all: "numerous archaea utilize low pH habitats, the most extreme examples being Picrophilus torridus and Picrophilus oshimae, both of which metabolize optimally at pH 0.7" - eoearth.org/view/article/160977 $\endgroup$ – Nicholas Shanks Sep 5 '13 at 7:00
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Terraform, shmerraform. The planet that would take the

least resources to make fully habitable

is the Moon. Even on Mars it would take centuries of extremely intensive and expensive work before you could walk outside without a spacesuit on. Functionally, if you are looking for a new place to live in the solar system, think in terms of enclosed spaces, unless you are thinking of your great-great-great-great-great-great-grandkids.

If you have a way of building suitably resistant inflatable domes over large areas, the line between terraforming and artificial environments can really blur. That seems like the most plausible place where a human will first stand unprotected, surrounded by vegetation, look up, and see stars. The one-sixth gravity and the proximity to Earth give you a huge advantage in achieving that on the moon. Compared to the cost of terraforming, we're talking peanuts, even if you did it over hundreds of square kilometers. I can't find a reference so far concerning domes, but this infographic at Space.com points out several of the things the moon has going for it.

Edit: Found a reference for a domed lunar city! The proposal is to put a dome over Shackleton Crater at the lunar south pole, 25 miles in diameter and 5000 feet high. Sounds good to me.

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    $\begingroup$ See my comment to Chris's answer. You can't terraform the moon. Also, the Moon isn't a planet. $\endgroup$ – HDE 226868 Sep 16 '14 at 0:42
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    $\begingroup$ But using domes isn't terraforming. $\endgroup$ – HDE 226868 Sep 16 '14 at 0:45
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    $\begingroup$ "Fully habitable" does not mean to cover in domes, and no one else in the world defines terraforming like you did. $\endgroup$ – HDE 226868 Sep 16 '14 at 0:51
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    $\begingroup$ Many people who want to know about terraforming are really wondering what would be the most practical way to create a place in space that would feel roughly like the outdoors on Earth. Even if this asker isn't thinking that way, people who end up here often will. For that reason, i think it is important to look at that aspect of this. (Woops - sorry, she included Ceres, which is in the asteroid belt. Is it an asteroid, or a dwarf planet? At any rate, the moon is bigger...) $\endgroup$ – kim holder Sep 16 '14 at 0:56
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    $\begingroup$ Whatever. I guess it could go either way. Oh, and yep, Ceres is a dwarf planet. $\endgroup$ – HDE 226868 Sep 20 '14 at 19:58
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I'm afraid there aren't really any good candidates for terraforming in Earth's solar system. Mars and Venus were once quite similar to Earth naturally. The fact that they aren't now demonstrates that they would have to be artificially maintained over geologic time scales. Worse, their substantially different physical parameters would keep them from ever being truly Earth-like. Romantic fantasies aside, what would be the point of taking centuries if not millennia to build EPCOT Earths when artificial environments that better reproduce the conditions that we prefer could be built in years or decades. Built from carbon nanotubes, such structures could be hundreds of miles wide and thousands of miles long. Space enough for anyone I should think, and the people who build them would actually be able to live in them. If you want a project that stretches over generations, build such a space colony, attach a space drive and head for the stars.

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Mars is a no-go because it's core has cooled and stopped it's magnetosphere has collapsed exposing it to radiation and the problem that even if you could build an atmosphere, solar windows would constantly strip it away.

If you could achieve couple of epic engineering challenges Venus is possible;

1) First you would need to increase it's spin rate which would give it a magnetosphere, by somehow getting mercury to fly by and then lock in as a moon.

2) Second you would need to deal with it's atmosphere, now there all sorts of complex solutions proposed, but basically you need to dump a huge amount of hydrogen on it (e.g. from Jupiter), this would convert all the CO2 into oceans (80% coverage) and a 3 bar atmosphere.

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There is a site, Terraforming Wiki. Even if it is not written by scientists or supervised by an institution, it still contains some useful information about terraforming and very interesting points of view.

There are many things to take into consideration when it comes to terraforming. Here, I want to point out only 4 of them.

  1. Gravity is very important. A too low gravity will make a celestial body lose its atmosphere. If one day we create an atmosphere around the Moon, it will at some point be lost in space, even if that will take more then a human lifetime. This takes out of consideration smaller moons and dwarf planets.
  2. Water is the second most important. All inner planets and moons (except Earth) have a lack of water. If we don't bring water to them, they will become large deserts. By contrast, moons of the outer planets (except Io), if heated, will become ocean worlds. Also, other volatiles are missing where water is absent.
  3. Luminosity is the third constrain. Plants need a certain amount of light and also they need both red and blue light waves. I made simple experiments with plants (see here). Beyond the asteroid belt, plants still can grow, but I don't know if they can feed a human colony. Beyond Neptune, plant life is probably impossible.
  4. Chemistry of the target planet or satellite is also important. Some chemical compounds might be dangerous for life. Some substances can be transformed, others cannot. Venus has an atmosphere rich in carbon dioxide and sulfuric acid, but both compounds can be transformed. By contrast, Titan might have an internal ocean as salty as the Dead Sea based on Cassini findings. We still don't know how to desalinize a planetary-sized object.

If we look at these four constrictions, we can say that no celestial body is easy to terraform. Also, there are greenhouse gasses that can rise temperatures even for the distant Pluto to values suitable for life.

Unfortunately, there are many things we don't know. For example, we still don't know what is the amount of water available on Mars. Will it be enough to create an ocean? On the other hand, many of the moons of the gas giants seem to have salty planetary oceans, which might not be suitable for Earth life. We know that Enceladus has an alkaline ocean with a pH of 11 to 12 and that also Europa has alkaline salts dissolved in its ocean.

In conclusion, I don't want to point a specified planet or moon as the best candidate for terraforming. Only two things are to be said: that terraforming is very hard and expensive (much beyond our current technology) and that we need much more research data before pointing to a celestial body or another.

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