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This question is related to Deadly, Heavier than Air Gas, and Is a world with two different types of air possible? but has several different criteria. I've also looked at https://chemistry.stackexchange.com and not found anything

I'm building an alternate Earth in which, some time in the 18th or 19th century, the world was covered to a depth of ~100m by a certain gas.

I need a gas with the following properties:

  • Denser than air (necessary)
  • Will remain separate from the air above it (necessary, with a few metres of interface)
  • Gaseous down to at least 0°, preferably -50° or less (semi-necessary,would need to be gaseous at the very least in the daytime)
  • Not flammable (covering the planet in a flammable gas sounds like it would bring a whole new meaning to the phraseGreat Balls of Fire (optional, would settle for only flammable with difficulty - I don't want to watch the world burn)
  • Relatively safe (would only kill you by asphyxiation, unlike say Chlorine) (optional)
  • Contains Hydrogen or Helium atoms, which can extracted with c19th technology (alternatively, something else which can be extracted and burned as fuel) (optional)
  • Opaque or visible (optional)

Unfortunately, it's been a while since I last did any Chemistry and I can't think of any gases that quite fit the bill. The noble gases are out because they don't help with the fuel element, while gases like Benzene fail the 'not flammable' test.

Is there a real gas with the above properties, or will I have to go with Handwavium?

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    $\begingroup$ I'm pretty dang certain that you can't have a layer of hydrogen or helium below a layer of a nitrogen/oxygen mixture; in pure form, both of H and He are far lighter than any way of having N or O. You may need to relax that requirement unless you're going with just gaseous handwavium. $\endgroup$ – a CVn Aug 8 '17 at 17:08
  • $\begingroup$ Helium obviously is not possible since it is the noblest of all the elements. Please be aware that being flammable isn't a property of certain gases but rather of a certain mixture, e.g. with oxygen $\endgroup$ – Raditz_35 Aug 8 '17 at 17:09
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    $\begingroup$ Man, that last requirement might be a dealbreaker for you. I think there might be some gasses heavier than air that contain Hydrogen, but they would all be flammable with oxygen. There are none containing Helium, I don't think. The gas I would choose would be Sulfur Hexafluoride, which satisfies all your requirements except the last. $\endgroup$ – MozerShmozer Aug 8 '17 at 17:10
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    $\begingroup$ Apologies, by contains Hydrogen I meant contains Hydrogen atoms, not a mixture, E.G hydrocarbons. $\endgroup$ – walrus Aug 8 '17 at 17:11
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    $\begingroup$ @walrus Say which properties are a must and which ones are bonus (optional). $\endgroup$ – user9981 Aug 8 '17 at 17:32
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I have been pondering this one and I think I have a candidate. Fog.

• /Denser than air/ – water as a gas is not denser than air, but can exist at earth temperatures in equilibrium with its liquid phase. The presence of microscopic droplets of liquid condensing from and evaporating back into the gas makes the fog more dense than air. Additionally (as below) liquid/gas water will stratify out with cooler, denser air.

• /Will remain separate from the air above it / Gases are all miscible but still air will stratify by temperature. Cool air is denser than warm. Cool conditions are necessary for the above described equilibrium of liquid and gas phase. If the air is stratified by temperature, the water vapor will stay in the lower cooler layer. This is a ground fog. enter image description here

• /Gaseous down to at least 0°/ Sure. Gaseous water can also exist in equilibrium with the solid form: ice. On reading I learned the difference between ice fog and freezing fog. Ice fog is suspended crystals of ice in the air that just hang there. Freezing fog is supercooled water that turns to ice when it touches anything, like the road.
enter image description here Don’t go out driving in the freezing fog, worldbuilders.

• /Not flammable/ - water excels at being not flammable.

• /Contains Hydrogen or Helium atoms/ – plenty of H in H2O and extractable with primitive electricity tech.

• /Opaque or visible:/ Both!

I am liking this also because fog is ok for earth. Fogworld would be spooky and surreal. I could suspend my disbelief for a world where fog moved in to stay.

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    $\begingroup$ This is what you call "outside the box thinking". But now I want to know how you make 100m fog covering the earth at all times. This isn't part of the question, but it sounds like a much more interesting one $\endgroup$ – Raditz_35 Aug 9 '17 at 5:15
  • $\begingroup$ Strictly speaking fog is not gas, but very tiny droplets of liquid suspended in air. Water vapor is invisible. $\endgroup$ – L.Dutch Aug 9 '17 at 10:22
  • $\begingroup$ This is a very cool idea. In this case there appear to be several questions that answer @Raditz_35's question (see worldbuilding.stackexchange.com/questions/73652/… ), but if that weren't the case would it be good material for a follow-up question? $\endgroup$ – walrus Aug 9 '17 at 16:51
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    $\begingroup$ @L.Dutch: True, fog is an aerosol, but that's what makes the layering workable. As other answers explain, a gas just won't do that. Over a long period, anyway: there are temporary examples such as WWI gas attacks or the CO2 release from Lake Nyos: en.wikipedia.org/wiki/Lake_Nyos_disaster $\endgroup$ – jamesqf Aug 9 '17 at 17:44
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No, sorry. Gases by definition are miscible. There are no gases which (in the absence of a gravitational (or possibly electric) gradient will separate. On Earth the gravity is insufficient to even get CO2 to concentrate below the O2 and N2 (molecular wts 44, 32, 28 respectively).
Also, your desire for a gas which is a source of energy but is "safe" is another oxymoron. If it can be readily converted to another state and provide energy, it is quite unlikely to be "safe". Are you able to give a counter-example? (Keep in mind that wood, as saw dust, is quite explosive). Two gases come to mind which are dense and fairly inert SF6 and NF3. Both are big time greenhouse gases but not too toxic, iirc. You might be able to get some energy from them, I'm not sure what the reactions would be, possibly by reacting them with water...just a speculation.

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    $\begingroup$ More exactly, in gaseous mixes, there is a funny thing in physics. You can calculate the dependence of the concentration on the barometric height formula, but, for both gases induvidually, as if the other component wouldn't be there! So, a little bit of separation can be reached, but without high tech engineering it is not useful for anything. $\endgroup$ – Gray Sheep Aug 8 '17 at 20:08
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Sulfur Hexafluoride SF6

enter image description here

  • Denser than air (it is used as invisible water trick to make small objects float) enter image description here
  • non toxic
  • non flammable
  • invisible

Funny side effect:

As with all gases, the density of SF6 affects the resonance frequencies of the vocal tract, giving the voice a "demonic" quality when SF6 is inhaled

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  • $\begingroup$ I realized you had already posted this just after I posted my own SF6 answer. I'll move it to a comment here. $\endgroup$ – bendl Aug 9 '17 at 13:34
  • $\begingroup$ Sulfur hexafluoride is extremely dense, reacts with basically nothing, and is colorless and odorless and boils at −64 °C. It's so dense you can float an aluminum foil boat on it. It doesn't contain hydrogen or helium, but it's relatively easy to make - just expose sulphur to fluorine, the latter of which was first discovered in the 1700's (I think) and the former just sits around in rocks. Isolating fluorine is notoriously difficult, but I don't think you necessarily need to have it sitting around alone to get it to react with the sulphur. $\endgroup$ – bendl Aug 9 '17 at 13:35
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Your alternative Earth would have to have a radically different material composition from our Earth. There is a gas that might fit the bill. It is rare on our Earth, but if on yours it is very abundant then it could work.

The candidate gas is xenon.

Xenon has atomic number 54; that is, its nucleus contains 54 protons. At standard temperature and pressure, pure xenon gas has a density of 5.761 kg/m3, about 4.5 times the density of the Earth's atmosphere at sea level, 1.217 kg/m3

But let's look at the density of air.

Air density, like air pressure, decreases with increasing altitude. It also changes with variation in temperature and humidity. At sea level and at 15 °C air has a density of approximately 1.225 kg/m3 (1.225 x10−3 g/cm3, 0.0023769 slug/(cu ft), 0.0765 lb/(cu ft)) according to ISA (International Standard Atmosphere).

Source: Density of air

Air's density is 1.225 g/L. The density of xenon is 5.881 g/L. Xenon has approximately five times the density of normal air. Therefore, if xenon was an abundant gas in the atmosphere of a planet it could accumulate at lower altitudes. However, the miscibility of gases would tend to counteract the probability of a denser layer of up to one hundred metres (100 metres).

For more information about the noble gases.

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    $\begingroup$ This answer would be improved if you discussed how xenon fits the OP's requirements. $\endgroup$ – a CVn Aug 9 '17 at 9:32
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    $\begingroup$ @MichaelKjörling The answer was accidentally posted before it was complete. Hopefully this finished version will do better. $\endgroup$ – a4android Aug 9 '17 at 9:45
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    $\begingroup$ Better! But please don't invent your own acronyms for units. The acronym for "grams" (singular and plural) is "g", not "gms". Either use the acronym, or write the unit name in full. $\endgroup$ – a CVn Aug 9 '17 at 9:49
  • $\begingroup$ @MichaelKjörling I don't invent an acronym. Grams was usually abbreviated as gm or gms. Obviously I'm stuck in earlier academic paradigm. This is an abbreviation not acronym, by the by. $\endgroup$ – a4android Aug 9 '17 at 10:09
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There are a few elements in your specifications that don't make sense.

  • Gases mix with each other. The only way for you to have only a certain gas on Earth's surface (whether it's only the 100m the closest to the surface or not, it by not having other gases on the planet. Which is kind of hard to imagine in Earth, since most of the land surface is covered with lifeforms which emit various gases (O2, CO2, Ch3, etc...) and water evaporates all over the Earth - especially above oceans and seas.
  • You said you wanted a not flammable gas, that is also safe (only kill by asphyxiation) separated from air with a few meters thick interface between both layers. If there is a 100m thick layer of your gas devoid of any other gas, then whether it's flammable or not, it won't burn. Combustion also needs oxygen, without it there is no fire. There is no breathing either, so everything asphyxiates to death.
  • I'll live the density part aside. A gas density depends on its pressure, which depends on the amount of gas, the volume it's in and its temperature. What kind of gas it is isn't very relevent here.
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