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Last Tuesday, as every Halloween, I re-watched the Cowboy Bebop movie. For those who don't know, it is a sci-fi anime set around the solar system in an hypothetical future.

This question focuses on the "airwalls" that surround the cities on Mars. They seem to be there to keep the atmosphere from escaping the cities by blowing some air up. They are needed because obviously glass domes would be too big and would probably collapse (in addition to being an obstacle to spaceships).

We see one in the distance in the following shot (larger version available by clicking for all following images):

enter image description here

And more upclose here:

enter image description here

How would these "airwalls" be able to maintain a breathable athmosphere within them, while keeping almost space conditions outside, to the point that highways connecting different cities need to be enclosed in tubes?

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Since someone asked about the allowed tech level, this is a human society a few hundred years from today. Humanity can travel easily (superluminal speeds) within the solar system and colonize other planets. I would not know how to quantify this "level".

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    $\begingroup$ I refined this question through the sandbox (worldbuilding.meta.stackexchange.com/a/5552/8240) sorry if it still not perfect. $\endgroup$ – Federico Nov 3 '17 at 12:18
  • $\begingroup$ I wonder if this might better go to the scifi stack exchange. I took a look at it just now. No questions on hold! It was uncanny. In any case maybe people there will be familiar with the Cowboy Bebop world's backstory or auxiliary works set in this world that explore the airwalls topic. $\endgroup$ – Willk Nov 3 '17 at 13:03
  • $\begingroup$ @Will the thing is that I am not looking for a "in-universe Cowboy Bebop explanation", but a more abstract "what could make them work", and I thought that this would have been a more apt community for that. $\endgroup$ – Federico Nov 3 '17 at 13:15
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    $\begingroup$ Please describe your question in a way that does not require guessing or watching the movie. What are properties you are looking for? What tech level should be used? How large these need to be? Allowed power consumption? Etc... $\endgroup$ – Mołot Nov 3 '17 at 13:18
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    $\begingroup$ "keeping atmosphere from escaping" is requirement that leaves this question really broad. Allowing spaceships in and out too - and without seeing the movie we don't know how it was done. "obviously glass domes would be too big" - it is only obvious for those who knows the lore of the world. You gave no reason against domes. And so on... $\endgroup$ – Mołot Nov 3 '17 at 14:45
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I always assumed they were dual purpose. If you want to create an atmosphere on mars, you need to pump out a metric heck ton of gas. But if you do it around the edge of a city, maybe you're maintaining enough of an atmosphere inside the city to be breathable, but outside it's still going to take hundreds of years to produce enough atmosphere for the rest of the planet; Creating a pocket of livable habitat in the mean time while you're slowly terraforming the rest of the planet.

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If the city was constantly outputting atmosphere from some kind of generator, the heavier oxygen-nitrogen mix of gas would tend to gravitate to the surface. If you could create big blobs of atmosphere, and continue making it at a decent rate, you could push the Martian atmosphere away from yourself. The airwalls themselves could be the point at which the oxygen-nitrogen and Martian atmospheres mix. Although, knowing diffusion, and knowing that the oxygen-nitrogen air would be over a hundred degrees warmer than the Martian atmosphere, the CO2 from Mars' air would diffuse into your blob of air.

I would like to think that the airwalls are a kind of force field. The setting of sci-fi certainly permits that. Some kind of electromagnetic barrier bounding the atmosphere inside the force field dome. Force fields, as we know them, can be created. However, they would consume a monstrous amount of energy. Probably why we don't use them.

In the sci-fi scenario, I think it would almost be better to build a physical barrier. The gravity on Mars is like a third of Earth's, which lowers the constraints of most building materials. If they've got structures utilizing carbon nanotubes, they should easily be able to build a dome that covers the entire city. Minimal sheets of force fields could be used to cover entry and exit points along the physical dome.

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    $\begingroup$ The first paragraph assumes that the native Martian air pressure is roughly equal to the desired life sustaining air pressure. It isn't and the resulting WIND would spoil things. $\endgroup$ – amI Nov 3 '17 at 19:19
  • $\begingroup$ Surprised to learn force fields can be created! Link for additional reading would be welcome. $\endgroup$ – Willk Nov 3 '17 at 21:26
  • $\begingroup$ I hadn't assumed equal pressures at all. Like I said, you'd need to pump out a considerable amount of atmosphere to sustain a life-supporting pressure and temperature. All in all, it's massively wasteful. $\endgroup$ – B.fox Nov 3 '17 at 22:59
  • $\begingroup$ You're not going to find a wiki page on them or anything. They are hypothetically plausible. Create a very strong, toroidal field of magnetism and stick a bunch of star-hot plasma inside of it, while placing the person at the center of the torus. Anything passing into the field of contained plasma would likely be reduced to their elementary particles. The whole thing would shine like a mini sun, and the person inside would need considerable protection. The amount of energy required to build one exceeds its practicality. But it could be done ... $\endgroup$ – B.fox Nov 3 '17 at 23:04
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Sounds to me like a case of “it’s obviously too difficult to contain with something solid so we should use something intangible with more give in it like an air wall”. It doesn’t sound very likely at all, the best answer is probably to assume a lot of people are arm waving the problem away or some sort of advanced force field perhaps, but that is just speculative.

Some high pressure air jets directed inward would have some effect but would not be remotely practical or effective. It would be like trying to herd cats to keep all that air in. If by some means an air wall was created I imagine it would still pose an obstacle to air craft or space craft entering it.

Probably best just to build a physical barrier which would be much simpler (well incredibly difficult but relatively speaking much simpler).

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Consider the 1952 London smog event. A temperature inversion (colder air in London, trapped underneath a layer of warmer air) combined with an anticyclone prevented the air in London from mixing with the rest of the atmosphere.

In London, this caused the deaths of some 12,000 people, but on Mars it might be more helpful. Some devices in a ring around the city could superheat the Martian air above them, causing it to rise and form a lid trapping the colder oxygen-nitrogen mix below, and propel it into a continual anticyclone surrounding the city.

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    $\begingroup$ London was not surrounded by a low vacuum like Mars... $\endgroup$ – L.Dutch - Reinstate Monica Nov 3 '17 at 19:33
  • $\begingroup$ @L.Dutch: You're right, of course! That's a big problem here. I suppose if terraformers managed to first create an atmosphere with some bulk gas, this approach could be used to keep oxygenated air around the cities. $\endgroup$ – Nick Matteo Nov 3 '17 at 21:11
  • $\begingroup$ If you could terraform mars then the problem is already solved. $\endgroup$ – The Great Duck Nov 4 '17 at 0:25

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