Inverted Everest world

Question

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?

Henry Taylor · Answer

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!

chasly - supports Monica · Answer

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.

John Dallman · Answer

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.

Inverted Everest world

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