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Context for Question

The colonisation of the planet [ INSERT NAME ] was an enormous social experiment performed by the Terran Planetary Science and Terraforming Council; this previously-barren world had its orbit and gravity adjusted to suit human requirements, but was uniquely terraformed so as to create an ecumenopolis (much like Coruscant of the Star Wars universe).

Any criminals placed on death row within the justice systems of other human-colonised worlds instead had their memories irrevocably wiped, any diseases cured, scars healed (including missing limbs) and tattoos removed, and were sent to become the founding citizens of the societies on this planet. The purpose of this exercise was to test the sustainability of a planet-wide society developing without the natural world (including all plants and most animals, and fungi).

Their food was to be entirely vat-grown, using animal and some plant tissues, but that was the most they would ever see of either type of organism. Their language and the visual aspects of their culture would be expressed through small and large digital screens; if you were to give any of these people a piece of paper and a pencil or pen, they would not only not know how to use it, but they could not tell you what it was made from. They had never experienced actual plant fibres or extracts.

Now, the Question:

How does this society produce its oxygen? The nearest plants need to be on another planet (for the purposes of this social experiment), so is there some other artificial chemical process which could be used to extract O2 from CO2? Further, could these filter materials be periodically 'cleaned' (if you will), even if they might eventually run out of cycles? And when this happens, could they be recycled?

Also, I just want to specify that the memory wipes of the citizens are completely irreversible. I am not concerned about the ethics or reliability of this procedure for the moment.

EDIT 1: I would also like to point out that, at the point where the narrative is set, the number of local years (of that planet) since the first colonists arrived is between 300 and 500, with the number of days in these periods similar to that of Earth. A few generations have existed and died, and the population has spread out further among the pre-made skyscrapers of the ecumenopolis. (Also, the genetic diversity of the population is maintained by either the government or a scientific body.)

EDIT 2: These people need to be able to walk around the planet as we do (without pressure suits or gas masks). I am also assuming that water is kept in a large (yes, ocean-sized) reservoir on a local moon, and brought down to be used and recycled for a few years, before returning to the moon and being replaced.

EDIT 3: For the world of this narrative, normally I would place water reservoirs underground, but I wanted to make it clear to everyone here that ocean organisms and any solution involving large bodies of water were not a viable option. So, yes, I sent them to the moon. I can see how this would look in retrospect, and I'm sorry about that. I just didn't need solutions involving the planet's non-existent oceans, or any solution too reliant on a large body of water.

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    $\begingroup$ I wouldn't call interplanetary shipping of water sustainable. It's a massive waste of energy just to move anything back and forth even before you consider how heavy any significant quantity of water is. Stick with aquifers. $\endgroup$
    – Separatrix
    Mar 1, 2016 at 14:22
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    $\begingroup$ I predict certain doom. Without extensive training and expertise, a group of mind-wiped ex-cons are not likely to have the skills necessary to form a viable society on an alien world with little or no resources. Prison colonies have succeeded in the past (e.g. Australia) because the locations of these colonies happened to have plenty of natural resources. Dropping a bunch of ex-cons onto a planet that doesn't even have breathable air is just a very expensive death sentence. $\endgroup$
    – Darrel Hoffman
    Mar 1, 2016 at 14:37
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    $\begingroup$ I think Layna's point might be that the Great Oxygenation Event on Earth happened before there were plants, and (with not much doubt) was caused by cyanobacteria. So you don't need plants for oxygen. One simple way to work around the "no plants" restriction is to have big regions covered in vats of cyanobacteria photosynthesising away. But that might not be what the questioner wants to hear, since it might not be interestingly different from giant vats of plants doing the same thing. $\endgroup$
    – Steve Jessop
    Mar 1, 2016 at 17:31
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    $\begingroup$ The question makes no sense. We're told food is grown in vats, from animal and plant tissues. The CO2-O2 cycle is perfectly balanced. Respiration of a given quantity of CO2 requires the consumption of exactly the amount of sugars that were produced in the photosynthesis that replaced that quantity of CO2 with O2 in the first place. The vats, in order to produce food, must also liberate exactly the amount of O2 the people must inhale to digest that food. $\endgroup$
    – Monty Harder
    Mar 1, 2016 at 19:49
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    $\begingroup$ No fungi means no yeast. No yeast means no booze. A colony of ex-cons without booze? Survived 300 years? That, and on a more serious note, there's hardly a way to survive without oceans. Look at the "weather" that you find on planets without oceans. You have like 200°C during the day and -100°C during the night... water is the one big thing that makes a rock inhabitable. Not only because you need to drink, but also because it stabilizes temperature. $\endgroup$
    – Damon
    Mar 2, 2016 at 12:47

5 Answers 5

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One of the very basic things you need for complex Earthly life is water. Human need water to drink, taking baths, washing clothes etc. So the planet you are going to send the pioneers has to have a large body of water. Without this, it would be simply impossible to sustain a human settlement on any planet in the long term.

For the sake of this answer, let us say that the planet has only water content equal to pacific ocean and no larger. Now comes the question of whether the pioneers are living inside their mothership or out in the open?

If they are in the mothership, you have several options for oxygen production.

1- If the planet has carbon dioxide in its atmosphere, you can use sodium peroxide or sodium superoxide (Na2O2 and NaO2) to convert carbon dioxide into oxygen and sodium carbonate. Sodium carbonate can later be processed back into sodium oxide carbon through some electrolysis and heat processing. That is how they used to supply submarines crew with oxygen, before world war 2.

2- They can simply go on electrylosing water into hydrogen and oxygen if the water body on the planet is large enough to not be depleted by the removal of a few hundreds of thousands of tons of water per year.

3- They can grow plants inside the mothership so that the plants convert carbondioxide back into oxygen. You can hence setup a mini-carbon-cycle inside the mothership.

However, if the pioneers are living out in the open, there are few practical options.

1- If the pioneers are wearing space-suits all times (for pressure reasons or whatever), you can use the same approach as stated in method 1, where the pioneers go and get their suits oxygen supply refilled once it is depleted.

2- If the planet has a large enough carbon dioxide content in the atmosphere, you can go on sowing green plants there and actually convert it into a green planet like Earth.

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  • $\begingroup$ This is a good response, but I just want to clarify that these people are not to experience any significant part of nature. The conversion of this planet into a green planet is out of the question, for the moment. However, your answer has shown some unspecified parts of my question, which shall be now edited in. $\endgroup$
    – dw0391
    Mar 1, 2016 at 12:09
  • $\begingroup$ You can still use blue-green algae (aka Cyanobacteria) in the oceans for large scale oxygen production. These add no nature (as in aesthetic) to the planet's ecosystem, but are a very potent source for starting and maintaining the carbon cycle. $\endgroup$
    – Youstay Igo
    Mar 1, 2016 at 12:24
  • $\begingroup$ Thanks, but this world does not have any oceans. I was contemplating putting the reservoirs of water underground, but I wanted to make clear for the purpose of this question that ocean organisms were not viable for this planet, as there are no oceans. So, off to the moon they went. $\endgroup$
    – dw0391
    Mar 1, 2016 at 19:08
  • $\begingroup$ The solution of your problem involves creating a carbon cycle. Here on Earth, the carbon cycle goes through the green plants part which convert carbon dioxide into oxygen. You can develop electronic trees with solar panels in place of leaves, which absorb sunlight and charge the batteries. Then this stored energy is used for superheating carbon dioxide in the presence of catalysts, breaking it into carbon and oxygen. It is possible to build such units with our current technology, but I'm afraid they would be very expensive (subjectively speaking). $\endgroup$
    – Youstay Igo
    Mar 1, 2016 at 19:20
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    $\begingroup$ Oxygen sinks are a massive problem. On Earth, it took about a billion years before cyanobacteria finally "filled" all the sinks and oxygen started to accumulate in the atmosphere. Humans could do it a bit more efficiently, but nowhere near a few hundred years. Closed habitats are likely the only reasonably realistic option. Unless the planet had an oxygen atmosphere in geologically recent times, of course. $\endgroup$
    – Luaan
    Mar 2, 2016 at 8:59
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You have to consider the cycle:

You need energy to live, how does this work, you eat plant and use oxygen and in effect you breathe out CO2 and water.

What a plant does is, breathe in CO2 and take in water and energy (from sunlight) and uses this to make the plant you eat and O2 in the oxygen.

And this would be exactly what your vat grown food source would need to do. Take in CO2 and water, put in energy, exhume the oxygen en give the food to your people.

In effect your vat-grown food-source would be a plant-plant. Remember, to make food you need to put in energy in one way or another and transform sources without energy to sources with stored energy.

PS: You could import your food and your oxygen, but you would end up with a wetter and wetter planet with more and more CO2 in the air.

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  • $\begingroup$ Came here to say exactly that. Amount of O2 inhaled and CO2 exhaled by humans should be pretty balanced by amount of CO2 intake and O2 release by food-growing vats. Carbon in food needs to come from somewhere. $\endgroup$
    – Mołot
    Mar 1, 2016 at 14:00
  • $\begingroup$ Your vats also have to have sufficient surface area exposed to sunlight to capture enough energy to convert CO2 to C + O2, plus what's needed to sustain human metabolism. Or have the necessary energy supplied in some other way. $\endgroup$
    – jamesqf
    Mar 1, 2016 at 18:06
  • $\begingroup$ This is actually something I did not think of... Thanks. It could end up being one of the methods I use. $\endgroup$
    – dw0391
    Mar 1, 2016 at 19:09
  • $\begingroup$ You don't need to expose plants to sunlight, because you can route the solar power anywhere, really, by use of solar panels and lamps. $\endgroup$
    – loa_in_
    Mar 2, 2016 at 0:58
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    $\begingroup$ @jamesqf Yeah, but then why use plants to produce sugars and oxygen? They're incredibly inefficient compared to what we can make with machines (depending on the plant and environment, they capture about 3-6% of the incident energy). They do have some advantages, but for a civilization that can colonize other planets, and for people who are outcasts, machines are quite likely the better option. Even with the photovoltaics, you'd still get maybe 5-10 times as much efficiency as with natural plants. And you don't need to wait for a harvest :P $\endgroup$
    – Luaan
    Mar 2, 2016 at 9:03
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The core of the question:

is there [some] artificial chemical process which could be used to extract O2 from CO2 [?]

Yes there is. If you have an abundance of energy then it is easy to break up carbon dioxide into its constituent elements Carbon and Oxygen. Insert any carbon-free source of energy you like — massive solar farming, nuclear fusion, geothermal, hydro — and you can make oxygen out of carbon-dioxide.

Fun bonus: if the process is done right, and you have yet some more energy, that ultra-pure carbon might be made into diamonds. ;)

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Phytoplankton can live in the ocean: not plants, but algae and assorted microorganisms. People would not be aware of them without using a microscope, even if they did encounter them.

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  • $\begingroup$ This is hw the majority if oxygen on earth is produced. plants are only a minor contributor. $\endgroup$
    – John
    Jan 29 at 14:29
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Does the society need an oxygen production facility? Per wikipedia, "In the present equilibrium, production and consumption occur at the same rate of roughly 1/2000th of the entire atmospheric oxygen per year". It means that without oxygen production, Earth atmosphere would last for some 2000 years, being human breathable for maybe half of that time span.

Just create an oxygen atmosphere during terraforming and leave it there. Assuming the biosphere is replaced by the megalopolis, the oxygen consumption will go at about the same rate as Earth's, and it will last for maybe 1000 years before the first serious problems appear. More, if you pump more oxygen at the beginning.

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  • $\begingroup$ Thank you, but I am looking for a sustainable, long-term cycle which might be useable. Although, this raises some interesting facts. $\endgroup$
    – dw0391
    Mar 1, 2016 at 12:17
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    $\begingroup$ CO2 levels would rise approximately the same absolute number O2 levels would fall, which in relative terms means much faster rise. CO2 levels would become toxic for human very fast, therefore unless you are somehow disposing it, your atmosphere will not be breathable soon in spite of having enough oxygen (remember Apollo 13 Co2 filter hack? - that was needed exactly for this reason). $\endgroup$
    – Suma
    Mar 2, 2016 at 6:29
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    $\begingroup$ Creating an oxygen atmosphere on a world that never had free oxygen takes a lot of time. You have to fill in all the oxygen sinks first - cyanobacteria on Earth took about a billion years to do that. Humans could probably do it more efficiently, but still nowhere near to hundreds of years. $\endgroup$
    – Luaan
    Mar 2, 2016 at 9:06

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