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The method of terraforming I'm thinking of is hauling the minimum required load of compost to the world to be terraformed, dump it on the planet's surface and seed it with plants that are hardy in an oxygen-poor atmosphere. Then, the plants convert CO2 to O2 and sugar which they use to grow, resulting in more compost on every iteration of the cycle of life. Like desertification but backwards. Follow-up passes by the terraforming crew will seed the world with less hardy plants.

Now, what I'm wondering is this: can this process create a habitable zone on a planet, at least on warm-enough latitudes while the terraformed area expands mostly on it's own? In particular, I'm thinking of a frontier world with mostly animal transport where a homestead will have an emergency cabinet with oxygen masks for the family. As for livestock, stall- to barn-sized oxygen tents, or caulking and crossed fingers, and maybe a second-hand oxygen regulator, for those without.

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    $\begingroup$ The atmospheric composition of a planet's atmosphere will be uniform with respect to the permenent gases such as Oxygen nad Nitrogen. So it wouldn't be possible to have a high oxygen zone and a low oxygen zone. $\endgroup$
    – Slarty
    Commented Oct 14, 2017 at 19:42
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    $\begingroup$ @Slarty Atmosphere will only be of uniform composition if it is in a chemical equilibrium which it specifically will not be during terraforming. Speed of diffusion is limited by speed of sound which is not that fast on a planetary scale. So if the terraforming is concentrated on a single area surrounded by mountains the oxygen concentration will be higher. No idea if enough higher to make a difference. I doubt it. Speed of sound will still be faster than photosynthesis. $\endgroup$
    – Ville Niemi
    Commented Oct 14, 2017 at 20:59

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As @Slarty said in comments, oxygen just doesn't stay put. So if the ${O_2}$ content is the main problem which the terraforming is addressing, the colonists will have to rely on the oxygen masks for a long time. There won't be a local partial solution.

However ...

If it's important to have a partially-terraformed planet in a useful way, does it have to be ${O_2}$? That is, can you assume that ${O_2}$ levels are fine, but that there is some other issue which is being terraformed? Example... let's imagine that there's plenty of oxy, but that the native life is all right-handed amino acids (we are lefties!) and unsuitable for human consumption. With this setup, we can have a "cleared zone" where the native life has been cleared and replaced with Earth-style stuff.

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Plants respire at night. This requires oxygen, same as animals.

They make oxygen during the day. But it diffuses away.

Your scheme is non-viable. You need a dome.

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The problem here is that oxygen is displaced by heavier gases, ${N_2}$ and ${CO_2}$ being the classic ones. So a region of heightened oxygen is unlikely to form in the first place since the rest of the atmosphere will be constantly displacing the ${O_2}$ from the area of production.

To create enough oxygen fast enough to have the effect you describe would require vast outputs of ${O_2}$ which you won't get from a natural forest, think more along the lines of the terraforming stations from Aliens pumping out megatonnes of cold ${O_2}$ that dissipates across the landscape and only mixes with the native atmosphere as it warms up. That would give you the geographically limited high ${O_2}$ you want but sorry not with a forest, the rate of mixing would be too great.

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  • $\begingroup$ so high enough in the mountains you would have more light oxygen and less heavy N2 and CO2? $\endgroup$
    – Henning M.
    Commented Oct 16, 2017 at 11:21
  • $\begingroup$ @HenningM. Maybe, Oxygen doesn't usually separate out in nature, Nitrogen displacement is usually a result of a temperature difference, cold Nitrogen displacing the warmer normal atmosphere including the Oxygen, Carbon Dioxide is naturally far denser than the rest of the atmosphere so in high concentrations it can separate and settle on rare occasions. Usually the atmosphere stays homogeneous and concentrated pockets of a single gas are the result of a local out-gassing event of some kind. $\endgroup$
    – Ash
    Commented Oct 16, 2017 at 11:43
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Earth has regions of high/low oxygen. As an example, climb a mountain. If I wanted to make a planet with more exaggerated zones, I would think of it in terms of those lines.

So it would become more a matter of atmospheric density rather than the concentration of ${O_2}$ being different in different places. Maybe the low-lands, in canyons and what-not would have enough ${O_2}$ because the air density is high enough but the higher one goes the less ${O_2}$ is to be had because, again, the air density changes at higher elevations.

As time passes and the terraforming goes on, more atmosphere is made and zones change accordingly.

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