Life on a planet with multiple gas layers

Question

Could a terrestrial planet have a stable set of layers of atmospheres, and could different lifeforms evolve within those different layers?

This question is inspired by the diversity found on Earth. For example, deep sea organisms evolved without light (and in some cases, without oxygen) and are quite different from other organisms at the surface, which in turns are quite different from flying or walking species.

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To clarify, I was thinking about a kind of swamp, where land-like creatures and plants living below a certain altitude would evolve in a different atmosphere. I though that maybe the gas could be "trapped" in a valley or isolated from the upper layer by the surrounding vegetation.

You may have guessed that I am far from being a biologist.

The question popped in my head while thinking about a possible science-fiction story. Tim has good arguments about why this is not likely to happen and I appreciate the reality-check. I also doubt that the kind kind of gas that could support life (See "Is it possible for complex life to evolve on planets without oxygen?") would also be heavier than, say, oxygen, and form a distinct layer.

But nonetheless, if you can find creative yet plausible ways about how it could happen, please share your thoughts. For example, I like the idea that a more massive planet might be suitable for the existence of multiple layers, as well as the possibility that weird forms of life can evolve in that kind of environment (see HDE 226868's answer).

Answer 1

I had previously made an argument for the separation of gas into different layers based on molecular weight; I now believe that argument is just plain false. Therefore, I'll only keep the second half, relating to life on said planet.

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Could different types of life form in different layers? Well, the cheating way out would be to say that there are incredibly large plateaus or columns sprouting high into the sky in certain areas. That would mean that there could be animals that walk on land - even 10,000 feet up! But in this scenario, it would be better to have creatures that permanently stay in the air - ones that don't need anything solid underneath them.

These creatures will have to be aerial - which gives us a surprising degree of flexibility. Sure, we could go with wings, but that's boring. You can attach wings to a lot of things and call it a flying creature, but who likes that? I'm thinking about something more like an airship. Picture a pufferfish. Got that image in your head? Now remove all the bits that it needs to be in the sea. Quickly give it a respiratory system - that we can modify - and set it on the ground. Yes, on the planet's surface.

We'll make it like a polyp. Say it formed from a sort of seed dropped after two of these odd aerial pufferfish hooked up. It fell to the ground and began to burrow into the soil. Over time, the seed begins to develop - it's like an embryo in a womb. Eventually, the creature grows into something that looks like our friend, the pufferfish.

For now, it can breathe the gases on this lower layer. But for some reason, it needs to go to the upper layer of the atmosphere, where there are a different combination of gases. Like a tadpole, it changes its respiratory system, then (unlike a tadpole) inflates its pouch like a zeppelin or a rising submarine (perhaps it creates a weak vacuum by expelling the lower layer gas), and rises into the air until it reaches the upper layer, at which point the gas inside is no longer lighter than the gas around it.

Weird? Oh, yes. But it could work. In the upper layer, it could move around by maybe using small fins to steer. Perhaps it takes in energy via photosynthesis. It lives, falls in love, reproduces, dies, and falls back to the surface. In the lower layer(s), creatures that are completely different could have developed. The two zones only have tangential interaction, yet can support different types of life.

Answer 2

TL;DR: There are a number of problems with this concept, and while none of them individually kill it the combination makes it highly unlikely, certainly for anything larger than bacteria.

What differentiate bands?

In the ocean it's easy, surface light loses power fast underwater whereas pressure increases. The depths are dark high pressure environment. This gives life an incentive to seek out suitable conditions.

What would cause bands to form?

So in atmosphere we already know that light gets through just fine. What else could form bands? In theory you could get regions like our ozone layer where certain gasses collect but in practice the entire atmosphere is constantly churning, moving, rising and sinking and rotating. Distinct bands with drastically difficult conditions would be unlikely to form and just as unlikely to be stable if they did.

What would life in this band live on?

In the deep ocean you have hydro-thermal vents and you also have a lot of rich resources falling from above. For example entire micro-communities form around a whale carcass when it falls tot he sea floor. There is no equivalent process in the atmosphere though, resources fall out of the atmosphere not into it.

How would life stay in the band

Microbes already live in the atmosphere, carried in raindrops and potentially even seeding rain clouds to make rain. Anything larger though has to constantly fight gravity. Assuming a band of suitable conditions existed it would need to find that band and somehow stay in it.

Conclusion

You can see why the atmosphere is very different from the ocean in this regard. It's hard to see how the sort of environment you are envisaging could form.

Answer 3

Biochemistry-induced differences

Hal Clement authored a nice novel called The Nitrogen Fix in which he supposed the emergence of organisms that converted the nitrogen and oxygen of Earth's atmosphere into nitrogen oxides. These compounds are very similar in overall thermodynamic stability - so much that any hot engine can create nitrogen oxides while a cooler catalytic converter can mostly (but not entirely) convert them back. For your planet, I'm going to suppose that the bacteria do N2 + 2O2 -> 2NO2, and they are common in the soil of a high pressure, high temperature area (let's say, the lower reaches of the Red Sea after it is dammed for power generation, at maybe 1.4 atm of pressure). The NO2 is heavier than N2 or O2, so it diffuses out slowly. When it does diffuse out, there are organisms in the air (perhaps introduced as part of a planetary protection program to avoid ending up like a Hal Clement novel) that convert them back to N2 and O2.

Pilgrims who visit the site believe the line of volcanic activity at the bottom of the Red Sea and Rift Valley is a symbol of Creation, the same phenomenon as the biblical Mount Horeb. They wear extensive protection against toxic NO2 and the acidic environment created by nitrous and nitric acids in the area. There is a freshwater acid swamp at the bottom of the former sea, with a layer of brine at the bottom, that is fed by runoff from agriculture fed by the desalinated water created by the power project. Strange cave slimes grow all around the shore under the faint brown gleaming of the distant Sun. Only those who have walked the long road and meditated by the shores of the swamp are worthy to learn the strange secrets concealed in the final depths. They return shaken, changed, the emissaries of something larger than themselves.