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Is it possible to have a world that meets the following conditions?

• There are two types of air (in term of its gas mixture)

• Each type of air has its own creatures which cannot survive in the other type of air

Let’s not focus that much on the creatures but rather on the reasons for the existence of two types of gas mixtures.

Would it require some kind of (half) closed environment (like a cave) for one of the mixtures in order to stay separated?

If not, what could be the reasons for the two types of air to don’t mix with each other (or just to a minor degree)?

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    $\begingroup$ For a real-life example, see the Lake Nyos disaster in which 1700 people were killed by a flood of heavier-than-air CO2. $\endgroup$ – DaaaahWhoosh Nov 18 '16 at 15:52
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    $\begingroup$ Water could technically be considered a kind of air that is below its melting point.... $\endgroup$ – nijineko Nov 19 '16 at 1:46
  • $\begingroup$ Regardless of whether it's possible in reality, at least one author has two different kinds/layers of air -Tolkien! Vista was the "normal" air, covered by a layer of Ilmen which came down to the ground in Valinor and farthest east, and through which the Sun, Moon, and stars travelled. $\endgroup$ – Spencer Aug 8 '17 at 23:22

14 Answers 14

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Gas can be kept separate by density. The gases in question would exist in layers, much like the layers of an onion. This is in fact the case with Venus's atmosphere.

The separation could be affected by many factors, such as air pressure, temperature, etc. So, for example, a layer of gas with a slightly different composition could exist below sea level, yet the composition might become very different at above sea level, etc.

Some creatures would float in the one layer, and not be able to descend/ascend into the other.

Depending on your geography you could have plateaus, or mountain peaks which are basically inaccessible to some species due to the atmosphere changing at that altitude. Similarly, the species living at those altitudes might not be able to descent because they find the air below poisonous/toxic.

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    $\begingroup$ By the nature of gas, gases with different density will mix up (slowly through diffusion, faster when there is some turbulence). They will not form layers in the long run. $\endgroup$ – jknappen Nov 18 '16 at 16:23
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    $\begingroup$ Even really heavy gases like 1,1,2-Trichloro-1,2,2-trifluoroethane (CFC-113) were going up to the stratosphere and causing damage to the ozone layer there. $\endgroup$ – jknappen Nov 18 '16 at 17:27
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    $\begingroup$ yes the air would "mix" but the two different layers could be different enough to prevent much interaction of the things that live in them. It could even be something as being above or below the cloud layer.you are going to need some big height diffrences in the ground, venus has a layered atmosphere. $\endgroup$ – John Nov 18 '16 at 18:03
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    $\begingroup$ @Brian - the ocean was more or less what I had in mind when I wrote my answer, actually. $\endgroup$ – AndreiROM Nov 18 '16 at 19:13
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    $\begingroup$ With any level of circulation (natural weather) gases are not going to remain layers by density. $\endgroup$ – paparazzo Nov 18 '16 at 21:46
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Rather than two completely different air types, it would be easier to set up a "normal" atmosphere that covers the entire planet which Creature-type Alpha breaths with no problem.

The other type of atmosphere would have additional compounds that are required by Creature-type Omega and is poisonous or debilitating to Alpha creatures.

You are correct that you will need to limit the area of one of them to prevent the atmosphere from completely mixing. Here are some ideas on what could be a cause of the localized atmosphere. (I'm not sure how "real-world" an answer you are looking for).

  • One "narrow-ish" continent has heavy volcanic activity along east and west coasts with massive mountains (super-volcanic Andes). The heavy gases released settle into the interior of the continent. Concentrations of this gas are much greater nearer the volcanic vents, but periodic burst of the gas can sweep out into the interior to prevent settlement by A creatures.
  • A widespread "plant" releases an airborne neurotoxin with its spores. Omega creatures have adapted both an immunity to the toxin as well as an ability to metabolize the toxin. This toxin is also highly physically addicting, leading to death should the Omega creatures not regularly be exposed to it.
  • Alien terraforming engines are spewing a specific mix of vapors that are deadly to native lifeforms but are vital for the alien ecosystem. Alien vapors are limited to the areas around the engines.
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So a very dense/heavy gas like sulfur hexafluoride has the ability to support the weight of objects, like a boat floating on invisible water.
So you could have creatures that live on the surface, and creatures that bob on top of the gas and roost on the mountain tops that rise above the gas.
I don't know what kind of bio chemistry it would take to survive breathing that gas exclusively.

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  • $\begingroup$ @jknappen Hmm, right... Now I'm trying to think which gas I was thinking of. Thanks for the catch. $\endgroup$ – AndyD273 Nov 18 '16 at 16:35
  • $\begingroup$ Other dense gas options might be Tolulene, Benzene, Krypton, and Xenon. And Radon, if you are into that. $\endgroup$ – kingledion Nov 18 '16 at 16:43
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Another possible scenario would be where an Earth-like planet maintains an orbit around its star in which one side never has daytime and the other never had nighttime (like Earth's moon; forgive me for forgetting the correct astronomical term used to refer to this type of orbit).

The dark side of the planet can contain volcanoes, gas geysers, etc. spewing all kinds of harmful stuff and poisonous gasses, and be lit and warmed by said volcanic activity and a minor binary star. Terrain would be a variation between hell-like badlands and snowswept plains. Some of earth's lesser-know creatures survive in these kinds of environments. Or invent your own form of cordyceps u., that's a scary bastard...

On the daylight side of the planet, the sun's UV rays and other radiation, etc. could break down the harmful chemicals that waft over from the dark side of the planet, making life comfortable for guys like me. A chemistry major would know what types of gasses and compounds would do this.

This way, the planet's water cycle can continue as we know it on earth, feeding life and the weather, etc. Something along these lines could work in a fictional story, being plausible enough to convince 99% of readers to suspend their disbelief.

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  • $\begingroup$ Tidally locked, is when half of the hemisphere of an celestial object always facing the parent body. $\endgroup$ – Hendrik Lie Nov 21 '16 at 16:23
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Closed rooms certainly help in maintaining an atmosphere with a different composition.

Since entropy wants to mix up gases of different composition, you need to think of a mechanism to maintain the two different kinds of atmosphere (like steady wells of CO2 by volcanic activity, or some kind of strange biological or technological activity).

EDIT: On earth, there are two different types of "atmosphere" possible for bodies of water: One oxidating, containing lots of solved oxygen, and one reducing, dominated typically by H2S and methane.

The mechanism of maintenance is skeched: Photosynthesis is only possible in the upper layers of a water body where there is sufficient light. Dying organisms sink down and their decomposition uses up oxygen. When there is no strong mixing mechanism, the lower layer of a water body becomes anaerobic. This is the case, e.g., for the water body of the Black Sea. The Oceans have strong mixing mechanisms (the thermohaline circulation) preventing anoxic zones even in the deep sea areas.

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Yes, actually air composition varies slightly even on Earth depending on the place. In example if there is "fog" you already have 2 layers of air with different compositions, one hot wet layer and one cold dry and of course also the fog/clouds.

In general any mixture of gas that have one or more components condensate or precipitate at different temperatures can be kept separed by different temperature zones. Of course there may be gases which are just more heavier (in example Radon tends to be inside caves, undeground, even though Randon can't react chemically so it can't be breathed). In example if your world is divided roughly into highplanes and lowplanes the different altitude can be enough to keep 2 different mixtures separated.

However to give an idea of which mixtures can act like that you have better to wait for an answer from someone expert in gas mixtures.

Also if some air compound is unstable it will be present only near the source of the compound, in example there is some poisonous gas produced by a forest, some species may have evolved to breath it, and thus can live only in those forests.

At this point probably you should think to some mixtures that can be potentially be brethed and then seek for a natural way to have both of them but separed.

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  • $\begingroup$ At first aid course we were taught that CO lays on the ground, first thing to do, hold breath, open windows and put people on the ground on a higher surface. Was that a fake? $\endgroup$ – GameDeveloper Nov 18 '16 at 16:51
  • $\begingroup$ Seems the first aid course was wrong. CO is actually "lighter" than air, my bad for not checking it. O_O Removed from answer ^^ $\endgroup$ – GameDeveloper Nov 18 '16 at 16:56
  • $\begingroup$ I removed that piece ^^. If downvote was yours what about removing it? :) $\endgroup$ – GameDeveloper Nov 18 '16 at 17:23
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Not on a living planet. Weather is how we get fresh water on dry land. Circulation is how CO2 and oxygen are exchanged.

Separate layers with no exchange of air layers would invalidate all plant and animal life we know today.

No circulation then air layers could separate by density. Just don't see how you sustain life in an environment like that. Nor create an environment with no circulation from weather.

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  • $\begingroup$ On the other hand, there could be in-layer circulation, but that is way more complex. $\endgroup$ – Daerdemandt Nov 19 '16 at 11:12
  • $\begingroup$ Weather is primarily driven from delta-t or delta-p, right? Wind is essentially energy moving from high t/p area to low t/p area. The only way not to have that would be ... what? Heat Death? $\endgroup$ – Michael Blackburn Nov 20 '16 at 2:05
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All gases are miscible; there's nothing like the oil vs water situation. They would eventually homogenise. The only way to have an ongoing difference is if a component is being produced in one location and destroyed in the other, maintaining the gradient.

@nijineko No idea what you even trying to think about here, but if water was below its melting point it would be solid, i.e. ice.

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Option 1: Density (water and air) Option 2: Polarity (oil & water)

Density is the most straightforward hard-science approach. You would have a first layer of life at lower elevations, and another layer of life at the higher elevations. This is actually exactly what we have on Earth, where we call the boundary of environments "sea level." The only difference being the two environments in you world are both gasses, rather than one liquid environment and one gaseous.

The other option is that one of the gasses exerts polar (electromagnetic) waves, and the other doesn't, you would have pockets of each gas cluster together, like attracting like, and repulsing the other. This is why oil & water separate. Water is polar, and attracts itself. If you leave oil & water in a container they separate with oil on top because of density, but the initial separation is due to water's polarity. If they were the same density, you would get something that looked more like a lava lamp.

Now, perhaps the planet did not have enough gravitation such that both gases tend to escape, or perhaps the two races are competitively colonizing an environment that had no existing atmosphere, so some sort of handwavium-based technology could initiate a new "pocket," or hold pockets together. You'd then have something resembling blobs of mercury on a table, with empty space in between.

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    $\begingroup$ I don't think the polarity separation would work for gases as it works for liquids. $\endgroup$ – Paŭlo Ebermann Nov 18 '16 at 21:15
  • $\begingroup$ As other people have mentioned in comments, two gasses of separate densities don't naturally separate in the same way that a gas and a liquid do. They might be separate temporarily but over time they will mix. $\endgroup$ – sumelic Nov 19 '16 at 3:58
  • $\begingroup$ Um, Venus? en.wikipedia.org/wiki/Atmosphere_of_Venus $\endgroup$ – Michael Blackburn Nov 20 '16 at 2:11
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that situation slightly exists on Earth. There are many mountains high enough that normal mountain climbers need oxygen tanks to breath and risk death if they don't use oxygen. Populations living at high altitudes in Tibet and the Andies have adapted to the thinner air and can function better than lowlanders. And if they go don to sea level the denser air can be bad for their health.

Of course Human highlanders have only had thousands of years to adapt to thin air while some planet and animal species have had millions of years to adapt to it. They should flourish better than humans in the thin air and suffer more when taken to sea level.

The situation is more noticible in water. A fish that is fine in a fresh water river will usually die after a while if it swims down to the salt water ocean. Different levels of the ocean have different temperatures, oxygen content, salinity, etc. and many marine organisms ill die at different depths than their noraml habitat.

In science fiction there are a number of fictional planets where the air is breathable for humans only a some specific altitudes. Humans may only be able to survive in deep canyons where the air is thick enough or high mountains where the air is thin enough. And the same may go for some native lifeforms. On a gas giant planet like jupiter there may be several completely separate ecosystems of air borne life at different altitudes and pressures.

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All gasses mix together, as noted by Bloke Down The Pub. However, consider what does vary from area to area: temperature, pressure, and admixtures such as humidity.

You could have some essential substance whose soluability varies greatly with other factors, in the same manner as humidity, but is more complex/interesting than water.

You can also have equilibreium reactions that are affected strongly by pressure, temperature, and humidity. So, while excess water is not a problem since life likes water, you could have complex molecule A that turns into B instead in the presence of water (or pressure, temperature, or combination). B can be toxic to life that handles A, and vice versa.

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Several ideas:

  • You could have pockets of denser gasses underneath lighter ones. On an earthlike planet, an isolated cave would probably be the easiest way to do this. Say geological activity is constantly releasing the heavier gasses to replenish the caves. You could also go the Venus route, where a much lighter gas like oxygen or helium floats above the much heavier gasses near the surface
  • On the more extreme end, you could try and introduce some sort of natural centrifuge. Perhaps on some enormous gas giant, an endless spinning storm causes the heavier gasses to exist on the periphery, while the lighter ones remain on the inside. Such storms do exist on our own gas giants, although I don't know how much of a separation they actually cause.
  • You could also try separating them using the electromagnetic force. Maybe some process is constantly ionizing one gas but not another. The ionized gas is then attracted by the planet's magnetic field or some other source of magnetism, separating it from the unionized gas. Such a system probably wouldn't be very conducive to life, however.
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Our atmosphere is already a mix of gasses. We only use the 20% that is oxygen. Maybe you could evolve other species that breathe Nitrogen instead?

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Maybe use water instead of air? There are creatures that can only live in sweet water, other only live in salt water, some only in salt/sweet water-mix and some can live in more than one "water type".

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