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I want to make a world with a great deal of variation in habitability in terms of temperature and pressure. Some areas would be habitable; others less so, some would require protective clothing and some might not be accessible without pressure suits / protective vehicles.

The plan is to start with an approximately Earth sized world with roughly 1g, but different surface features and vastly less ocean. I want a world with a range of altitudes from the highest mountains to the abyssal plains with most of the comfortable habitable areas on the mountain tops and their immediate surroundings.

The atmosphere would consist mostly of carbon dioxide, oxygen and nitrogen but would be predominantly carbon dioxide, however given that carbon dioxide is heavier than oxygen or nitrogen I would expect to find a higher proportion at the lower altitudes.

The net result being high pressure, temperature and carbon dioxide concentration in the low lands and a gradation to lower pressure, temperature and carbon dioxide levels in the highlands and an interesting variety of environments in between. So a little like mt Everest in reverse liveable at to top but not at the bottom.

The proportion of the gases in the atmosphere and total pressure at the lowest point can be adjusted to fit as required, but unprotected humans must be able to live comfortably outside at high altitudes, uncomfortably at lower altitudes and not for long at the lowest altitudes.

Is this sort of world possible and if not how else might it be adjusted to give this effect?

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    $\begingroup$ Now this is a cool question. Isn't there some math that rationalizes planetary mass to maximum atmosphere? How do the mountain tops become temperate and arid when there's less water on the planet? Wouldn't that presuppose constant rain (or at least incredibly high humidity) at those altitudes? $\endgroup$ – JBH Jul 19 at 15:07
  • $\begingroup$ "...most of the comfortable habitable areas on the mountain tops and their immediate surroundings." How exactly does this differ from Earth? Consider California's Central valley vs the Sierra, the Andes vs the Amazon Basin, most of India vs the Himalayn foothills... Humans (e.g. scuba divers) can tolerate about 3X sea level pressure for fairly long periods, as long as they don't ascend quickly, so heat is likely to be your ruling factor. Of course that varies with latitude as well as altitude. $\endgroup$ – jamesqf Jul 19 at 17:34
  • $\begingroup$ Also, you are not going to find significant differences in atmospheric composition with altitude, since gasses mix too well. (Other than water vapor, which will condense out as the temperature decreases with elevation. So your lowlands would be either very hot & humid - Amazon Basin on steroids - or hot & dry - Sahara likewise. You might look at the Messinian Salinity crisis, when the Mediterranean Sea largely dried up, creating a basin several km below sea level: en.wikipedia.org/wiki/Messinian_salinity_crisis $\endgroup$ – jamesqf Jul 19 at 17:41
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On Earth, the composition of air is pretty much constant (apart from water vapour) up to about 10km altitude. Water vapour varies because of the temperature variations, since it tends to freeze out. So I don't think your variable composition with altitude works. The reason for this is simply that the atmosphere gets well-mixed by temperature differences (manifesting as weather).

If you take an Earth-like planet, and simply give it more atmosphere, then oxygen toxicity will start to affect people at low altitudes. The Wikipedia article on the high-altitude death zone has some useful numbers.

As an example, if you give your planet three times the mass of atmosphere, then the pressure at any given altitude will be three times higher, and the "partial pressure" of oxygen will be likewise three times higher. That gives you an Earth-normal atmosphere at the peak of a mountain the height of Everest, which looks right. People can venture down to the height of Everest base camp in reasonable safety.

But at sea-level, with an atmospheric pressure of 3 bars, and oxygen partial pressure of 0.6 bar, oxygen toxicity of the lungs will set in after a day or so. Nitrogen narcosis will also start happening in a mild form, impairing reasoning and unfamiliar tasks.

If the carbon dioxide proportion is like Earth's, the partial pressure at sea-level will be equal to about 1200ppm in Earth's atmosphere. This has been observed to have negative effects on thinking, but less than the nitrogen effects. If you have more carbon dioxide, that will get worse, but greenhouse gas heating isn't localised: it's slow enough that it applies to the atmosphere as a whole, due to mixing.

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  • $\begingroup$ That's an encouraging start. Given that oxygen and nitrogen molecules are similar in mass I would expect the difference in concentration at altitude to be very small. However a carbon dioxide molecule is a fair bit heavier so I would expect it to be slightly more concentrated in the lower levels of the atmosphere, that said it might not be that much and any difference would undoubtedly be swamped by the difference due to changes in pressure alone. $\endgroup$ – Slarty Jul 19 at 15:41
  • $\begingroup$ So oxygen and nitrogen work well in your model, but what about carbon dioxide? I suspect humans can tolerate a fair bit perhaps 1000ppm at the top of the mountain meaning much more on the plains (due to pressure differential) and consequently a lot of heating down there due to the greenhouse effect. $\endgroup$ – Slarty Jul 19 at 15:44
  • $\begingroup$ @Slarty: Explaining the mixing, and the CO2 effects. $\endgroup$ – John Dallman Jul 19 at 16:33
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Some areas would be habitable

By what? Humans? The local fauna? The local flora?

The fact is that Earth-like plant life would flourish in the swampy lowlands. The CO2 would be heaven for them and they would spread everywhere. They would eventually produce oxygen in such large amounts that it would change the composition of the atmosphere as happened on Earth. This would allow oxygen breathers to evolve.

I suppose the problem is creating a stable atmosphere like the one you suggest.

Is this sort of world possible ...

I think that the answer is that it would be unstable.

and if not how else might it be adjusted to give this effect?

If you waived the "habitable" constraint then there might be an answer based purely on physics. I'll leave that to the meteorologists and the physicists.

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Yes, your planet could exist.
Since we are worldbuilders, let's use Venus as a starting point...

Aside from the addition of sulfuric acid clouds, Venus is almost perfect, with a thick layer of carbon dioxide covering and insulating its planetary surface. Venus's highest mountain is called Maxwell Montes and it is 6.8 miles tall compared to our Mt. Everest which tops off at 5.4 miles. There is even a little free oxygen in the form of atmospheric ozone.

On the negative side, the atmospheric height is only a quarter of that of Earth. Venus is also a little on the scrawny side, with a planetary mass of 4.867 × 10^24 kg compared to Earth's 5.972 × 10^24 kg. It is also a lot closer to the sun, which when combined with it's atmosphere's insulating characteristic, makes the planet's current temperature a little warm for your usage.

None of those problems are insurmountable during the planetary creation lottery. It is easy to believe that a sol-equivalent solar system exists out there in which a slightly plumper Venus-like planet attained a slightly higher orbit. If its planetary composition also contained large quantities of a strong base compound, like Sodium Hydroxide, the sulfuric acid portion of the atmosphere might neutralize out and provide us with some water in the process.

Then all we need is some blue-green algae (maybe from panspermia) to produce the oxygen. From there, a non-turbulent atmosphere and a little gravity is all you need to let the atmospheric gases settle into layers. If the oxygen layer happens to settle out near the top of its highest mountains, that is just a wonderful coincidence.

Your planet is very possible and planets like it have probably evolved naturally in solar systems throughout the universe. More locally, your planet design might be part of our Venus's future, if our descendants survive long enough to become terraformers, aka real worldbuilders!

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  • $\begingroup$ Venus is an interesting example. But the top of the mountain needs to be habitable so the partial pressure of carbon dioxide can't be that high 1% might be manageable 10% probably not ok or not for very long. Also the partial pressure of oxygen needs to be much higher than on Venus $\endgroup$ – Slarty Jul 19 at 16:10
  • $\begingroup$ Have you checked that blue-green algae are net oxygen producers? "Blue-green algae photosynthesise during the day - adding oxygen to the water - but consume it at night. This means oxygen levels can be very low in the early morning and can suffocate fish and other creatures. When the bloom has subsided, bacteria causing the decay can also remove large amounts of oxygen." cdn.naturalresources.wales/media/686163/… $\endgroup$ – chasly - supports Monica Jul 19 at 16:16
  • $\begingroup$ @Slarty, Good Points! Whatever is going to live on your planet will need to be more carbon dioxide tolerant than base-model human. In my current work in progress novel, the characters are homoaquaticus having been genetically altered to live in a water-filled generation ship. Don't assume that all problems need to be solved on a planetary scale. A little genetic tinkering can fix a lot. $\endgroup$ – Henry Taylor Jul 19 at 16:26
  • $\begingroup$ @chasly-reinstateMonica, Interesting! No, my research into blue-green algae began and ended with a single google search, "where did earth's oxygen come from?". The answer said "blue-green algae" so that is what I went for in my answer. If your referenced study is accurate then I guess the origin of earth's oxygen is back to being an unanswered question. Easy to solve in my answer. Just replace "blue-green algae" with "net-oxygen producing blue-green algae". Just because our current crop of algae is so selfish that it consumes all that it creates, doesn't mean that all algae do. $\endgroup$ – Henry Taylor Jul 19 at 16:33

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