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I'm making a world where you live in the air, as the surface is not survivable (like Venus). I'm going to leave out the story of the planet, as it's not important.

Anyway, could there be a planet with different layers in its atmosphere? For example, the bottom layer, closer to the surface would be dense, heavy gas (or a mixture of gases), while higher up, there is a layer of lighter gas (or a mixture of gases), something like Earth's nitrogen and oxygen breathable atmosphere.

Is this even possible, and if so, what would the transition between the layers be like (smooth and gradual, sudden, etc.)?

Also, for a bonus question, what would the most likely gases in these layers be?

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  • $\begingroup$ Sounds like a gas giant. Would you be amenable to an idea like that? $\endgroup$ – HDE 226868 Mar 5 '16 at 21:25
  • $\begingroup$ @HDE226868 I was thinking about that but unfortunately I need actual land and mountains for this world. Thanks for the idea though! $\endgroup$ – Niko Mar 5 '16 at 21:37
  • $\begingroup$ this is totally possible and happens IRL; unfortunately, i dont know enough about chemistry to talk about the second part of the question with any real authority so im not going to post an official answer yet. $\endgroup$ – Duncan Urquhart Mar 5 '16 at 23:36
  • $\begingroup$ The planet your are describing is Venus. en.wikipedia.org/wiki/… $\endgroup$ – user20498 May 5 '16 at 15:24
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Yes

But,
there are many things working against this forming - especially for a terrestrial world. Here are the issues that spring to mind:

Chemistry

You can subdivide chemicals using many different strategies. I'm going to subdivide them by calling them:

  • reducers (fuels)
  • oxidizers
  • inert

If you have an atmosphere with a high quantity of oxidizers (like on Earth), then reducing gases (like methane) added to the atmosphere don't last long. They react quickly and are flushed out.

So really your atmosphere can only contain inert (like $N_2$) & oxidizers, like Earth or inert (like $He$) and reducers (like $H_2$ & $CH_4$), like Jupiter.

Fluid Dynamics

Fluids (liquids and gases are both considered fluids) tend to mix. There are two primary mechanisms for this:

  • Diffusion
  • Mechanical mixing (usually by turbulent boundaries between the layers)

The primary gases of the Earth's atmosphere are:
$$\begin{array}{|c|c|c|c|} \hline \text{Gas} & \text{Fraction} & \text{Atomic Mass} & \text{Type} \\ \hline \text{Nitrogen } N_2 & \text{78%} & \text{28} & \text{Inert} \\ \hline \text{Oxygen } O_2 & \text{21%} & \text{32} & \text{Oxidizer} \\ \hline \text{Argon } Ar & \text{1%} & \text{40} & \text{Inert} \\ \hline \text{Carbon Dioxide } CO_2 & \text{<<1%} & \text{44} & \text{Inert} \\ \hline \text{Water } H_2O & \text{Up to 1%} & \text{18} & \text{Inert} \\ \hline \end{array}$$

Note the absence of compounds considered reducers.

Despite the wide range of atomic masses, the Earth's atmosphere is generally well-mixed. This is due to wind. In some locations with volcanic venting you can get pools of (invisible) unmixed $CO_2$ gas. Animals (including humans) that wonder into these pools die from asphyxiation.

Carbon dioxide, being about 1.5 times as dense as air, caused the cloud to "hug" the ground and descend down the valleys, where various villages were located. The mass was about 50 metres (160 ft) thick and it travelled downward at a rate of 20–50 kilometres per hour (12–31 mph). For roughly 23 kilometres (14 mi) the cloud remained condensed and dangerous, suffocating many of the people sleeping in Nyos, Kam, Cha, and Subum.

List of candidate gases

From lightest to heaviest (in my list). The greater the difference in the gases masses, the more likely you'll get differentiation.

$$\begin{array}{|c|c|c|} \hline \text{Gas} & \text{Atomic Mass} & \text{Type} \\ \hline \text{Hydrogen } H_2 & \text{2} & \text{Reducer} \\ \hline \text{Helium } He & \text{4} & \text{Inert} \\ \hline \text{Methane } CH_4 & \text{16} & \text{Reducer} \\ \hline \text{Ammonia } NH_3 & \text{17} & \text{Reducer} \\ \hline \text{Water } H_2O & \text{18} & \text{Inert} \\ \hline \text{Neon } Ne & \text{20} & \text{Inert} \\ \hline \text{Nitrogen } N_2 & \text{28} & \text{Inert} \\ \hline \text{Carbon Monoxide } CO & \text{28} & \text{Reducer} \\ \hline \text{Nitric Oxide } NO & \text{30} & \text{Reducer?} \\ \hline \text{Oxygen } O_2 & \text{32} & \text{Oxidizer} \\ \hline \text{Hydrogen Sulfide } H_2S & \text{34} & \text{Reducer} \\ \hline \text{Fluorine } F_2 & \text{38} & \text{Oxidizer} \\ \hline \text{Argon } Ar & \text{40} & \text{Inert} \\ \hline \text{Carbon Dioxide } CO_2 & \text{44} & \text{Inert} \\ \hline \text{Nitrous Oxide } N_2O & \text{44} & \text{Oxidizer} \\ \hline \text{Nitrogen Dioxide } NO_2 & \text{46} & \text{Oxidizer} \\ \hline \text{Sulfur Dioxide } SO_2 & \text{64} & \text{Reducer} \\ \hline \text{Chlorine } Cl_2 & \text{70} & \text{Oxidizer} \\ \hline \text{Krypton } Kr & \text{84} & \text{Inert} \\ \hline \text{Xenon } Xe & \text{131} & \text{Inert} \\ \hline \text{Bromine } Br_2 & \text{160} & \text{Oxidizer} \\ \hline \text{Iodine } I_2 & \text{254} & \text{Oxidizer} \\ \hline \end{array}$$

Light gases are dominated by reducing chemicals while heavy gases are dominated by the oxidizing chemicals (primarily the Halogens). Inert gases like $N_2$ and the noble gases are sprinkled through the mix.

One Possible Setup

As long as the reducing layers and oxidizing layers are separated by inert layers, you might be able to have all sorts of gases.

On possible configuration would be (with the Bromine layer as the deepest layer).

$$\begin{array}{|c|c|c|} \hline \text{Layer} & \text{Gas} & \text{Type} \\ \hline 1 & \text{Bromine} & \text{Oxidizer} \\ \hline 2 & \text{Krypton} & \text{Inert} \\ \hline 3 & \text{Earth's Atmosphere} & \text{Oxidzer} \\ \hline 4 & \text{Helium} & \text{Inert} \\ \hline 5 & \text{Hydrogen} & \text{Reducer} \\ \hline \end{array}$$

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I'll add two more phenomena to think about after Jim 2B's answer. Firstly, why are clouds flat bottomed? A combination of temperature, pressure and saturation of water vapor results in a threshold being crossed at the bottom of the cloud. The presence of the droplets can change the nature of the chemistry in the two regions. The droplets are far denser than the gas, but are small enough to remain buoyant (due to Brownian motion). There is also a new thermal exchange system in this region where added heat would evaporate the droplets rather than heat the surrounding gas. This suggests one tenuous possibility for making distinct layers play a role in life differences.

Secondly, when it comes to mixing, gas are notoriously difficult to prevent mixing. Liquids have a meniscus which pulls the atoms and molecules into the liquid, which can help separate layers of liquid. Hydrophobic behavior can add to this layering. This has no equivalent to my knowledge within gasses. This absence works against layering effects within gasses as are found in liquids.

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