It's amazing how complicated just the O2 cycle is. Basically, we don't yet how to do it without a whole planet being involved.

Like, plants do release O2 sometimes, but they also use O2 as fuel when they grow. Growing a plant requires light. On the earth that's easy, just put it in the sun. On Mars there's no atmosphere and no magnetic field, so if you just put a plant on the surface they'll die. So, you need to grow them underground in a mostly earth-like atmosphere at mostly earth-like pressure lit by artificial lights.

So, you plant a lot of plants deep underground lit by bright artificial lights. Then you need to supply the plants with a lot of water. Some of that water will be released into the air, but some of it will be incorporated into the plant's body. There's a whole water cycle that isn't yet fully understood.

What about the soil? On earth worms and other bugs break down leaf litter and other things into usable soil and bees pollinate many of the plants. So, do you ship up a bunch of bugs? You'd have to supply a whole ecosystem of them so they live in balance. You could go with hydroponics instead, but then you'd need a constant supply of nutrients for the plants, and given the amount of plant matter needed for just one human, that would be a huge supply of nutrients.

I'd love to see another honest, scientifically rigorous attempt at a biosphere project. Building a closed ecosystem on Earth is easy-mode compared to doing it anywhere else, but so far all the Biospheres have been failures. IMO until we can easily do it on Earth, we're nowhere near ready to do it in space, on the moon, or on another planet.

MARK WATNEY, SPACE PIRATE

That was the biosphere 2 experiment. Turns out it's very difficult to sustain the plants without the rest of the planet.

YouTube: Joel Creates - How many plants do you need to breathe? TESTED

Plants actually use O2 themselves a lot of the time, so we would still need to truck in a whole bunch of that stuff in. Also the amount of plants needed for just a single human is huge. Most plants are rather bad at producing O2. Most of it actually comes from algae living in water, not potted plants. The YouTube channel Joel Creates did an experiment with how much algae you would actually need to breathe. It's like a lot, a lot a lot really. Building some place on Mars or even in orbit that such a large amount of algae could happily live is pretty hard. Hell it's pretty hard on Earth, where you don't need to worry about temperature and pressure going out of spec or stuff like radiation. These days we do have pretty effective LED grow lights that prevent the whole thing from becoming too hot. From movies people think space is cold, but getting rid of heat is a big problem. With that much light blasting into the water, the temperature rises and the algae will die from that at some point. So radiating away all that heat is a must. On Mars or the Moon this is easier as the surrounding rocks could be used as a heat sink. The actual real hard part is not just building this, but building it in a way that can support human life for a long time. Systems such as these are chaotic in nature and often suffer from cascade failure modes. If a little thing goes wrong and some of the algae dies, it often cascades into a full failure where all of it dies off. So there would need to be many smaller systems, isolated as much as possible to prevent cascading failures. The system would also need to be modular enough so it would be easy to disconnect a module, completely clean and sterilize it and put it back into use. With staggered phases applied as to not have large swings in output. As these systems would be rather large in scale and have many different complex parts, a high level of automation is required. And we haven't even touched on getting all of this constructed somewhere and have it bootstrapped with enough water, with the right stuff in it and none of the wrong stuff. Enough reliable energy and nutrients to feed it all and transport living algae there to kick it all off. As far as I know nobody has ever gotten close to anything like this on Earth, let alone in space or on a place like the Moon or Mars. It would be a project that rivals the original Moon landings.

It might sound like a simple enough concept and it is how we are currently living on this planet, so it should be possible. However keep in mind our planet has had huge swings in temperatures and atmospheric composition. There were many many times in Earth's past where humans could not survive the conditions and we evolved here.

I drew a factory with the smoke stacks making a u-turn and stuck into the ground. Seemed like a good way to keep pollution from going into the air.

There isn’t enough CO2 for that either. Mars’s total atmospheric pressure is way less than 1% of Earth’s. Mars is closer to the Moon in terms of atmosphere than it is to Earth!

You need to keep it down because CO2 is heavier than O2

Congratulations, you're thinking like a billionaire!

Or were at 12.

Gonna need a lot more plants