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Imagine a big planet, gravity quite strong, 1.5 times earth. Irregular surface, rocky etc... (though flora and fauna will come into it later). It has a very thick atmosphere, drained off by a twin planet, even bigger, which is now a small gas giant.

Is it possible that - the air that is left could be held in craters? The gravity holds it low, in a shallow dense layer. In pockets of holes etc? Could the atmosphere above maintain a helium/hydrogen/methane mix? Or would it most probably just thin out - as per our atmosphere and maintain the same mix but thinner?

Sorry, a few questions there. I guess I'm after..

  1. Could the concept of oxygen pools work?
  2. If so, could there be a higher layer atmosphere made up of lighter gasses?
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    $\begingroup$ The oxygen in the atmosphere would oxidize the hydrogen and methane, forming CO2 and water. $\endgroup$ – ckersch Jun 16 '15 at 14:38
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    $\begingroup$ If you want a breathable atmosphere at low levels but not at high levels, you want a small planet, not a large one. Because a small planet with lighter gravity can have a much larger vertical range of land. If Mars had a thin oxygen atmosphere, it could be breathable at the lowest points (the Hellas basin) while the tops of the big volcanoes were still close to vacuum. $\endgroup$ – Mike Scott Jun 17 '15 at 9:13
  • $\begingroup$ @Mike Scott great to know. I'll have to adjust my world. Luckily the description of the planet only exists in a few lines. Its the topology that runs throughout the story. Thanks! $\endgroup$ – JimDiGriz Jun 17 '15 at 9:16
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In theory yes. In reality no

Edit : In theory also No

The molar mass of each elements tells the story in theory: Oxygen 31.99 g/mol. Methane 16.0425 g/mol Helium 4.002 g/mol Hydrogen 2.016 g/mol

If they were liquid this would tell us the answer. However as a gas the molecules move around individually. This is what makes them a gas and is called Brownian Motion. This would cause the gas' to get mixed up any way.

Any wind or other atmospheric disturbance would mix things up further.

On an actual planet/moon then there is no way this could work.

Edit: Thanks to @samuel for pointing out that I have forgotten most of my secondary school chemistry

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    $\begingroup$ Even if the crater is quite deep? $\endgroup$ – o0'. Jun 16 '15 at 10:49
  • $\begingroup$ @Lohoris that is my question as well. $\endgroup$ – JimDiGriz Jun 16 '15 at 10:54
  • $\begingroup$ Actually, @Wil Selwood - if we take Methane out of the picture, there is a much larger disparity in the mol mass between Oxygen and Helium/Hydrogen. So even with strong winds, would it be possible for a human to breathe at lower levels? $\endgroup$ – JimDiGriz Jun 16 '15 at 10:56
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    $\begingroup$ Wind would still blow into wide/deep craters, for features more like lava tubes I think you would end up with an effect like blowing over a bottle to make a whistling noise which would mix up the gasses again. The trouble with a high hydrogen atmosphere is that it tends to get lost due to solar winds, evaporation (Not sure that's the right word but I don't know a better one) and reactions with the oxygen to form water. The earth is thought to once have had a mainly hydrogen atmosphere very early in its existence. $\endgroup$ – Wil Selwood Jun 16 '15 at 11:06
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    $\begingroup$ Nothing moving including the molecules themselves. Are you not familiar with Brownian motion? Even theory says this won't work. $\endgroup$ – Samuel Jun 16 '15 at 20:19
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If you put two gases into a closed container they will mix, even if not stirred, this is due to the nature of the gas molecules and their movement. Temperature will play a role in the speed at wich the gases mix, and any mechanical agitation (besides thermal movement) will increase this mixing speed. What you can have is (if provided with a source of molecular oxygen) temporary pools of oxygen that over time mix with the rest of atmosphere. There is a mixing energy that must be provided for two things to mix, thats called entalpy (that specific entalpy i dont remember the name in english), that energy is used to move molecules around, but, for gas molecules to mix, the energy is small. A lot of factors play a role on how gases mix, thermal agitation is one. You cannot prevent a gas molecule from wandering far away from the pool, and thats how the gases mix.

This is different from liquids, the miscibility/imiscibility of liquids is related to their polarity/apolarity and molar masses, but polarity plays a larger role. Another factor when dealing with liquids is that, even if they dont have specific form (they assume the form of the recipient they are placed on) they have definite volume, while gases dont. Thats why some characteristics of gases are very different from the characteristics of liquids.

There are exceptions. But the general rule is this.

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While you may think that heavy gases push lighter gases up, this is not what physics is like. Instead, you can consider each gas on its own and see how a pure athmosphere of that gas gets thinner as we move away from the surface. This happens (for each gas individually) exponentially if the temperature is constant (Boltzmann statisics!), but the exponents differ - so that a lighter gas spreads out further than a heavier gas. The total "air" pressure is then just the sum of the component pressures (this all is correct essentially because gas is so thin that virtually no interaction takes place). It is only by this difference that at some levels one gas is more prevalent and at another level another gas. That being said, oxygen pools will not form any more than they do on Earth.

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Could the concept of oxygen pools work?

Yes, but they would be largely transitory, like rain. Most weather mixes the atmosphere quite well, but under some circumstances oxygen pools would form for days or even longer. Perhaps you have oxygen collectors people have to run, and, like rain in the desert, people conserve it. If the atmosphere has pressure, you don't need full pressure suits, you just need little nose cannulas for outside work, and houses which aren't very leaky if you don't want to wear them all the time.

If so, could there be a higher layer atmosphere made up of lighter gasses?

Sure, and we already have that - the Ozone is made up of lighter gases, while down here we have heavier gases. While mixing does occur, the reality is that the concentrations change as you go higher, and you find lighter gases more prevalent, and heavier gases less prevalent.

The weather conditions required for atmospheric pooling might be similar to a phenomena quite well known in cities that are essentially big bowls. It's called an inversion:

https://en.wikipedia.org/wiki/Inversion_(meteorology)

It's not going to give you exactly what you want, but a little fudging here and there and you could make this work for a world you have in mind if your audience isn't very picky.

In particularly still areas with no wind, such as in mineshafts, carbon dioxide pooling is a significant issue that has to be actively managed, so this does already happen, it's just not common in large open areas subject to weather patterns.

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  • $\begingroup$ I'm not usually keen on having rain in my dessert, but I understand desert flora really likes rain. :) $\endgroup$ – Frostfyre Jun 16 '15 at 15:24
  • $\begingroup$ Actually Ozone (O3) is heavier than most atmospheric gases - molecular weight 48 versus 32 for O2, 28 for N2, 40 for Argon. It stays high in the atmosphere because it is produced there, and decays before it can mix in the lower atmosphere to plain oxygen. $\endgroup$ – Oldcat Jun 16 '15 at 17:39
  • $\begingroup$ Thanks @Adam Davis - yeah, the more I hear about this the more I realise that perhaps this might work conceptually for my story, but it's really going to be speculative fiction. Even having temporary pool of oxygen would work. But I'll just have to adapt the story to fit. $\endgroup$ – JimDiGriz Jun 16 '15 at 22:44
  • $\begingroup$ Raining on a dessert is bad (makes it inedible), but raining in a dessert is outright impossible (if you have a building made out of candy stuff, it's no longer a dessert). On the other hand, rain in a desert, while infrequent, can happen. Would you please fix this typo of yours? $\endgroup$ – John Dvorak Jun 17 '15 at 11:26
  • $\begingroup$ @JanDvorak Fine, but you owe me a dessert for taking this one away. ;) $\endgroup$ – Adam Davis Jun 17 '15 at 11:41
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Pools will not settle out. If you carefully fill a depression with heavy gas and don't stir, it will stay that way for a while. Especially if the gas is being continuously emitted at a small rate at the bottom, as is the case for $\mathrm{CO_2}$ pools on Earth.

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Lets suppose that somehow the oxygen actually did pool, would it work?

What's at the interface? Hydrogen and oxygen. Unless one gas really dominates (for example, deep sea divers sometimes breathe an oxy-hydrogen mix with so little oxygen that it won't support combustion.) all it will take is one spark.

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  • $\begingroup$ There's also methane in the atmosphere that should stay between the layer of oxygen and the layer of hydrogen. The resulting interface would be... pretty ripe for combustion, too. Suggests an interesting plot though: you have discovered a really cool planet that you really want to live on, but the atmosphere is highly flammable. How would you colonise such a place? The villain could be a habitual smoker. $\endgroup$ – John Dvorak Jun 17 '15 at 11:22
  • $\begingroup$ @JanDvorak To colonize the place set it on fire, come back when it's cooled. $\endgroup$ – Loren Pechtel Jun 18 '15 at 1:14
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This is somewhat similar to how you are able to perform the "invisible fishtank" trick, where you have a more dense gas in the fishtank and balance less dense materials on it. Provided the upper layers of the atmosphere are less dense than the oxygen in the craters, they will rise to the top of the atmosphere wilst the oxygen sinks to the bottom.

Another example would be trying to float certain materials on water. Wood will float as it is less dense than water, but a stone will sink as it is more dense.

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    $\begingroup$ The Sulfur hexafluoride trick? $\endgroup$ – Scott Downey Jun 16 '15 at 10:17
  • $\begingroup$ I can't remember the specific gas used but I vividly remember the MythBusters episode on it. $\endgroup$ – Universalerror Jun 16 '15 at 10:31

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