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In another question, I asked how airships could be made more viable. The first (and so far only) promising solution I came up with is to increase the density of air.

In the real world, air (on Earth) is composed mainly of nitrogen (~80%) and oxygen (~20%). I'd like to reduce the nitrogen content in favor of a heavier gas.

Unfortunately, most heavy gases seem to be toxic in one way or another, although both Krypton or Xenon seem to be ok(-ish); Radon would be the next step in the noble gas category, but it's radioactive. Changing the air composition from a (simplified) 80% nitrogen, 20% oxygen to 50% nitrogen, 30% xenon, 20% oxygen would raise the density from about 1.25kg/m³ to about 2.68kg/m³, roughly doubling the buoyancy.

Hence my question: what would the effects of reducing the Nitrogen percentage in air in favor of a heavier gas such as Krypton or Xenon be?

Addendum: I since read this question, which has an answer that states "xenon makes an excellent general anesthetic, so let's not adopt an atmosphere which renders us unconscious". I'm open to simply ruling that this property does not exist in my world (perhaps humans and other creatures developed an immunity) if Xenon turns out to be the otherwise best option.

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  • $\begingroup$ Does it need to be an elemental gas mixture? Alternatives might include Halofluorocarbons perhaps - non-toxic (except as they displace oxygen) and really dense. $\endgroup$
    – Escaped dental patient.
    Commented Jan 22, 2021 at 12:27
  • $\begingroup$ @Tantalus'touch. I'd prefer elemental gases, as "the world's atmosphere is composed of nitrogen, oxygen and ksadfjhklsdjfklsdjlfjsdkljfldsjlskjfklj" sounds a little weird. I do appreciate any input on non-elemental gases as well, though - if they work better than elemental gases, they're still an option. Plus, I could always give them a custom name. $\endgroup$
    – PixelMaster
    Commented Jan 22, 2021 at 12:36
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    $\begingroup$ Much better, just increase pressure to about 2.5 atm, while decreasing the proportion of oxygen to about 12%. There is nothing sacred about the atmospheric pressure of Earth; it could just as easy be 0.5 atm or 2 atm. $\endgroup$
    – AlexP
    Commented Jan 22, 2021 at 12:37

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A very likely risk is that the heavy gases would stratify at the lowest heights, displacing lighter gases and making it an anoxic environment.

A situation where normal atmospheric mixing is stopped is called inversion

Given the right conditions, the normal vertical temperature gradient is inverted such that the air is colder near the surface of the Earth. This can occur when, for example, a warmer, less-dense air mass moves over a cooler, denser air mass. This type of inversion occurs in the vicinity of warm fronts

Temperature inversions stop atmospheric convection (which is normally present) from happening

The formation of anoxic regions due to heavy gases happens already during limnic eruption:

A limnic eruption, also known as a lake overturn, is a rare type of natural disaster in which dissolved carbon dioxide (CO2) suddenly erupts from deep lake waters, forming a gas cloud capable of suffocating wildlife, livestock, and humans.

Once an eruption occurs, a large CO2 cloud forms above the lake and expands to the surrounding region. Because CO2 is denser than air, it has a tendency to sink to the ground, simultaneously displacing breathable air, resulting in asphyxia. CO2 can make human bodily fluids highly acidic and potentially cause CO2 poisoning. As victims gasp for air, they actually accelerate asphyxia by inhaling CO2 gas.

This would be a high risk in areas with low atmospheric mixing. Of course above those areas, due to the stratification, buoyancy would also be reduced.

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  • $\begingroup$ Dutch are you are aware of any such areas above ground? Earth's atmopshere is pretty mixy. The Lake Nyan eruption killed things but it cleared fast. Is there anywhere where gases stratify? $\endgroup$
    – Willk
    Commented Jan 22, 2021 at 13:38
  • $\begingroup$ @Willk I know of areas where seasonally, due to the temperature distribution, it can happen that atmospheric mixing stops causing for example smog and pollution concentration. $\endgroup$
    – L.Dutch ♦
    Commented Jan 22, 2021 at 13:41
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    $\begingroup$ @Willk Geography can play a big role. I first learned about inversions when living in Pittsburgh, where the hills and valleys make temperature inversions not terribly uncommon. The 1948 Donora Smog, for example, sickened tens of thousands over a period of several days, when air pollution got trapped near the surface due to a temperature inversion. Not sure that something like that could happen anywhere, but it can occur naturally under the right circumstances. $\endgroup$
    – Nuclear Hoagie
    Commented Jan 22, 2021 at 17:46
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    $\begingroup$ @Willk, Earth's atmosphere doesn't have much variation in component density. The lightest of the non-trace components, nitrogen, has an atomic mass of 28, while the heaviest, carbon dioxide, has an atomic mass of 44. Compare that to xenon's 131. $\endgroup$
    – Mark
    Commented Jan 22, 2021 at 23:25
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You would sound like Darth Vader https://www.youtube.com/watch?v=irz-diec-qg%3Frel%3D0

Although the effect is only really significant at very high densities.

There is a bit of a problem with the choice of gas, many gases at high concentrations have active biological properties. For example noble gases from Argon upwards possess anesthetic properties, though only significantly so for Xenon or higher.

How about just, you know, increasing the ambient air pressure, thus increasing air density?

If you make the atmosphere 95% nitrogen and 4% oxygen, at 5 bar (5 times Earth Sealevel pressure), then your atmosphere is still perfectly breathable by normal humans, and your airships get 5 times the buoyant force from their balloons. Whether lighter-than-air gas, or simple heated air, makes no difference. 5 times the air pressure means 5 times the air density means 5 times the lift force per volume of balloon.

For normal humans, 5bar is pretty much the limit. Nitrogen narcosis kicks in a bit below this, actually, but some measure of acclimatization is possible. The reduced oxygen content will ensure appropriate partial pressure of Oxygen for breathing.

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    $\begingroup$ What other effects would this increased atmospheric pressure have? I imagine it would increase air resistance, but that would probably be true for a heavier gas mix with Xenon as well. $\endgroup$
    – PixelMaster
    Commented Jan 22, 2021 at 17:39
  • $\begingroup$ If you have five times the air pressure, don't you need proportionately higher pressures (and thus densities) of lifting gasses? I was thinking of this, but I don't know if it would actually be any different. Any physicists want to comment? $\endgroup$
    – DWKraus
    Commented Jan 22, 2021 at 22:53
  • $\begingroup$ @DWKraus, the lift capacity comes from the difference in densities, not the ratio of densities, and so gets multiplied by five as well. $\endgroup$
    – Mark
    Commented Jan 22, 2021 at 23:42
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It depends on the circumstances of your world. If a high Xeon atmosphere world did exist (very unlikely) then any creatures that evolved in the atmosphere would not suffer anesthetic effects. If high concentrations of Xeon were somehow introduced (!) then any life might suffer anesthetic effects.

Xeon would probably be found at slightly higher concentrations in the lower Troposphere than say in the Stratosphere but general atmospheric wind, rain and convection effects would almost certainly ensure a good deal of mixing. Note that carbon dioxide concentrations do not vary in the atmosphere (https://www.jstor.org/stable/1934239?seq=1) despite carbon dioxide being a heavier gas than oxygen or nitrogen or argon.

One way to increase buoyancy would be to make the world cold. Cold gases are denser than hot gases. If the interior volume of the lifting gas were to be heated (by chemical, magical or solar means) its lifting capacity would be increased.

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  • $\begingroup$ I imagine a high Xeon atmosphere would indeed be unlikely (SCNR ^^). That being said, it's a fantasy world, so - even though it should mostly still follow the laws of physic in order to maintain an internal consistency - simply declaring Xenon to be common is not a problem. I'm aware it probably doesn't make a whole lot of sense from a chemical point of view, but I'll take the artistic freedom approach here. Anyway, I'll look into the temperature option - might be worth a shot, provided the difference is notably (and not just +5% buoyancy or so) $\endgroup$
    – PixelMaster
    Commented Jan 22, 2021 at 17:23
  • $\begingroup$ Well it depends how much you want to heat the gas. Can all be reasonably calculated using the ideal gas laws - seems to work for hot air balloons. And yes by all means if its fantasy then fill your boots! Alternatively you could introduce what I call "minimal magic", that is keep everything as per our world except for one thing (as bit like assuming lots of Xenon) but ensure that not physical laws are broken. I think I was reasonably successful when I tried it here: worldbuilding.stackexchange.com/questions/191893/… $\endgroup$
    – Slarty
    Commented Jan 22, 2021 at 21:19
  • $\begingroup$ Cold would work. but your airship would not! To double your lift, you would need to, basically, halve the temperature. The absolute temperature. So 150K (-190f) would be a balmy summer day. $\endgroup$
    – PcMan
    Commented Jan 23, 2021 at 7:11
  • $\begingroup$ I think given the proposition the airship would have worked fine. The proposition (Heavenium) is the problem. Temperature is not a solve everything fix but it might help. If it was -30C 243K out side and even 30C 303K inside the envelope that would provide a useful amount of lift. And you could make it warmer still. $\endgroup$
    – Slarty
    Commented Jan 23, 2021 at 10:28
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I agree with the answers that suggest givining your planet a denser atmopshere. I will link to calculations suggesting that a breathable atmosphere could have up to about five times the sea level pressure of Earth's atmosphere - possibly much more.

You might also want to consider giving you planet a higher or lower surface gravity, whichever will tend increase the buoyancy of gases which are lighter than the atmosphere.

Of course every element or compound which might possibly be present in an atmosphere will become toxic if it is present in too great a concentration. Even Oxygen, vitally necessary for life, becomes toxic in excess amounts. So if you increase the concentration of every probable and improbable element and compound to the maximum that humans can tolerate, you will find an absolute upper limit to the density of atmosphere at the elevations where humans (and beings with the same requirements) live.

Habitable Planets for Man Stephen H. Dole, 1964, 2007, discusses the environmental requirements of humans.

And the 1964 edition is online:

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf[1]

On pages 13 to 19 Dole discusses the atmospheric requirements of Humans. On page 15 Dole states that the normal atmospheric pressure of oxygen at sea level on Earth is about 149 millimeters of Mercury. The overall sea level atmospheric pressure is 29.92 inches of Mercury, or 759.968 millimeters of Mercury.

On page 19 Dole says:

To summarize, then, the atmosphere of a habitable planet must contain oxygen with an inspired partial pressure of between 60 and 400 millimeters of mercury and carbon dioxide with a partial pressure roughly between 0.05 and 7 millimeters of mercury. In addiiton, the partial pressures of the inert gases must be below certain specified limits and other toxic gases must not be present in more than trace amounts. Some nitrogen must be present so that nitrogen in combined form can find its way into plants.

So carbon dioxide would be a very bad choice to increase atmospheric pressure. According to table 2 on page 16, the approximate upper pressure limits for nitrogen and argon would be about 2,330 and 1,220 milimeters of mercury. So a breathable atmosphere could possibly have a total of pressure of 3,950 milimeters of mercury of nitrogen, argon, and oxygen, which would be about 5.197 times the sea level pressure of Earth's atmosphere.

I note that Dole believes that helium and neon could be present at even greater pressures in a breathable atmosphere, but the upper limits are rather uncertain.

And of course newer research may have changed the safe limits of various atmospheric gases.

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