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I'm writing a graphic novel on the premise of a world that has a liquid or semi-liquid atmosphere, separate from a true liquid sea. But since I don't want 'Magical Underwater Adventure!' I've decided (tentatively) on a 'gaseous semi-liquid with non-newtonian-fluid-like physics' - i.e. under low pressure, it takes on more gas-like properties, and vice versa.

(From what I understand a gaseous NN-fluid is technically not possible, because there’s no ‘shearing’ involved… but I'm just looking for something with that particular, similar effect. Because of what I think it will result in. And the idea is kind of cool.)

For practical purposes, however, I want it to mostly maintain the visual and chemical characteristics of an Earth-like atmosphere (burnable, breathable, stable, insulates, transparent, etc.) You know. So you can see more than 100 meters or so at most. Also, I want it to be practical to roam around 'normally', run, jump, fall, throw stuff, shoot arrows, etc. Though some fudge is acceptable there.

The ideal is to have a pseudo-sciencey basis or maybe justification for things, and artistic license can come in after that, for visual/practical purposes. (Considering it's a visual media.)

I'm mainly struggling to visualize the physics and fluid dynamics, though. More specifically, how it would affect/create weather patterns and phenomenon, climate, and the like. Beyond that, I'm looking at ecology; and what it might mean for developments of societies. But I'll probably split those topics off into separate questions, and then link the whole thing together. I've got lots of notes. Any feedback here would be awesome, especially if it helps me reasonably justify or create (or improve!) my ideal.

So I'll just go through what I've kind of worked out / guessed at so far:

  1. Basic Physics
    • At high pressures, and to some degree cold temperatures, it will become more viscous and dense; and at low pressures, it will become more gaseous and thin. (It could go the other way, but I don't know that it'd create the effect I want.)
    • I figure it will be somewhere in-between normal air and water in density, so it will create surface tension with either. i.e. True gasses (Side-note - where would you notably see these??) would be slowish bubbles, and water would behave as if in a slightly low-gravity environment, (more 'globby'? Look up water-rag in space videos.) and the gradient between sea and 'air' would be slightly less stark; depending on pressure and temperature, of course. Heavier materials would more or less function the same, except perhaps for large surface-areas and aerodynamic forms.
      • This will also (ideally) create a slip-stream effect easily, so you could run around normally; (or maybe it'd be harder at first, but life would be adapted to that.) but at a certain speed you get crouching-tiger-hidden-dragon type leaps; arrows would travel straighter and longer, provided they don't run into a current; and the like. Also, this may create a more pronounced 'contrail' effect on very quickly moving objects.
    • Sound will travel similarly to normal? I don't know. I'd rather this didn't impede the ability to communicate. (Note to self: Google "How does water affect sound-transmission"?)
    • It will evaporate at higher temperatures, however, so while burning it will sort of 'boil' and look like a rolling lava lamp of plasma as it cools upward. Think prototypical mushroom explosion. Less heat will be radiated, more light and pressure. (?)
    • It will freeze at a lower temperature than water will, instead just becoming more viscous; but it will also make an easier structure (with particles) for water to freeze in midair and on objects; resulting in delicate, near-invisible 'lattices' that fracture with shearing (i.e. weird noises and slightly refracted, broken visual patterns) which will eventually build up to larger ice crystals/object layering.
    • It can be saturated with both fresh and salt-water, depending...? Like a saline air. I'd just imagine this would have interesting effects?
    • Explosions would oscillate like they do underwater, if more quickly and 'rolling' more vertically; transferring much of the force to pressure-waves rather than to heat. (Owing to cavitation and a wall of pressure-increased density) Although, accelerant explosions may expand in a sort of 'fire-wave', I imagine. (I'd rather the sky medium not be a super-accelerant itself, even though it is more dense... though I can see this technically being a byproduct. But I'd prefer not to have a burning-sky apocalypse.)

Before I get into climates and such, I do have a basic states-of-matter question: What, really, is the difference between a gas and a liquid? I don't feel there is a static line to judge; just relative benchmarks. The reason I feel this is the case is while you might consider a liquid a fluid because it will (a) take the shape of its container (b) without expanding to fill it completely, like a gas would; this doesn't take into account the effect of air pressure on top of it. Effectively, atmospheric pressure is part of the 'container' keeping it in its current volume. In other words, in a vacuum, water will expand (boil) immediately to fill its container, i.e. fulfilling the second criteria. Also, consider when you 'pour' carbon dioxide (i think?) into a container and it will stay there, allowing for the 'magically extinguishing match!' party trick.

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If you had high pressure (really high pressure, >73 atmospheres) and slightly high temperatures (>30 C), with a carbon dioxide atmosphere (which is feasible), you would get an atmosphere that is made up of supercritical carbon dioxide, which is a weird gas-like liquid.

The atmosphere of Venus is supercritical carbon dioxide and nitrogen, while water planets would have possible oceans of supercritical water.

Though I somewhat doubt that people could survive in supercritical carbon dioxide, even with breathing apparatus... plants might evolve to thrive in it, though. There is a paper discussing the properties of supercritical carbon dioxide and the possibility of alien life in it: (WARNING: pdf)

ADDED AS AN EDIT: Whoa! I just found out that it is possible for divers to adapt to pressures of 100 atmospheres! So with a protective suit and breathing apparatus (maybe one that can extract oxygen dissolved in the supercritical carbon dioxide -- it readily dissolves water and oxygen) you could hang out in the stuff. And 30C is just in the high 80F.

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  • $\begingroup$ WolframAlpha indicates that this pressure can be achieved at about 0.47 miles of ocean depth. No big deal. $\endgroup$ – Green Sep 12 '18 at 2:03
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I used to live in Glasgow, Scotland, and that had a semi-liquid atmosphere most of the time. See also Manchester, England. It seems to encourage musicality, since if you can't go out you're more likely to practise your guitar.

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  • $\begingroup$ Is this a joke reference to smog, or what? It's unclear what you're trying to say, and it doesn't seem like an answer. $\endgroup$ – HDE 226868 Oct 25 '15 at 16:57
  • $\begingroup$ Yes, it's a joke reference to areas that get a lot of rainfall. $\endgroup$ – Max Williams Oct 26 '15 at 8:24
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One of the biggest differences between a gas and a liquid is density. So actually having basically a much thicker atmosphere, would be more toward living in the water. Sound will travel better farther, but the planet will be darker as more light is absorbed by a larger density of 'air'. arrows would not travel straighter longer, in actuality with more density you get more drag and it would reduce the effective range of all ranged weapons.

It would make a difference on terminal velocity. you would be able to fall farther with less chance of injury because of the friction of the extra density. Which means it would also slow your jumps making them less impressive.

You might also have to be careful of where you sleep, since on a cool fall night a hollow might turn into a pond and drown you!

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  • $\begingroup$ This is why I'm theorizing a non-newtonian-fluid-like shift in density; so the drag is decreased to manageable levels. As far as less light goes, I understand that, and I talk about that in part 2. I'll basically have to fudge that. But a justification would be better. $\endgroup$ – spicklesandwich Oct 21 '15 at 22:19
  • $\begingroup$ Each occurrence of "mass" in your answer should be replaced with "density" or appended with "per unit volume". $\endgroup$ – Samuel Oct 21 '15 at 22:22
  • $\begingroup$ @Samuel you are absolutely correct. Spent to long listening to PluralSight courses today... $\endgroup$ – bowlturner Oct 21 '15 at 22:51
  • $\begingroup$ @bowlturner At least it was consistent! $\endgroup$ – Samuel Oct 21 '15 at 22:54
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A gas, as you say, expands to fill its container -- but it's subject to gravitation, pressure, and escape velocity, which is good if you want a planet to have an atmosphere!

A liquid can be somewhat compressible, but does not "expand to fill its container". Boiling doesn't count. If you put water into an evacuated container, it boils and cools until it reaches an equilibrium point. At that point, the container holds some liquid water, with water vapor (a gas) filling the rest of the container.

Another answer mentioned the critical point and supercritical fluids. If you're working on a story like this, you owe it to yourself to find and read Hal Clement's Close to Critical. It's set on a planet whose atmosphere is, well, close to its critical point; several plot points hinge on the strange behaviors of such a fluid.

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