Life on a planet with multiple gas layers
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.
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).
This post was sourced from https://worldbuilding.stackexchange.com/q/2856. It is licensed under CC BY-SA 3.0.
1 answer
A planet could definitely have multiple gas layers. As was discussed here, gravity affects different gases differently, based on their mass. In other words, there would be more force between the Earth and an $\text{O}_2$ molecule than there would be between the Earth and an $\text{H}_2$ molecule. It's why lighter gases rise and heavier gases sink (the same goes for liquids), and why we can use hot air balloons, blimps, and airships to travel through the sky.
So let's say we have $x$ molecules of $\text{H}_2$ and $x$ molecules of $\text{O}_2$. Release them above a the surface of a planet with, for some reason, no atmosphere. Assuming the planet has enough gravity, the $\text{O}_2$ molecules should sink to the bottom and the $\text{H}_2$ molecules should rise to the top (I say "enough gravity" because the planet has to have enough gravity to hold onto the $\text{H}_2$ molecules). You'll see "layers" of gases form.
An Earth-like planet does not have enough gravity to hold a substantial amount of hydrogen and helium - again, see the question I referenced above. But a larger, more massive one might. And besides, the lighter gas doesn't have to be $\text{H}_2$. You could have any number (and type) of gases. But beware: they will only form substantial layers if there is a large mass difference between a molecule of each.
But 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.
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