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Suppose you have a planet covered in a deep layer of supercritical CO2, and animals evolved to live in it.

Discounting the ones who crawl along the bottom, would their method of locomotion be more accurately described as swimming, or flying? Or in other terms, is this layer better described as an ocean, or an atmosphere?

The density of supercritical CO2 is comparable to water--between about 40% and 110%, depending on how high you push the pressure. That seems to argue for "it's an ocean, and stuff will swim in it; animals will be streamlined like fish, use fins, and tend to float". But the viscosity is much lower; it's more viscous than air, but much closer to air than it is to water. That seems to argue for "it's an atmosphere; animals will be aerodynamic like birds, use wings, and fly".

So, which is it? Or is the truth some weird in-between thing?

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  • $\begingroup$ I've answered your main question (would animals in supercritical CO2 fly or swim through it), but it would probably be good to edit your question to focus more tightly on this. $\endgroup$ – walrus Nov 28 '17 at 17:13
  • $\begingroup$ Supercritical CO2 will not forms pools, streams, etc. I.e., It expands to fill its container like a gas. $\endgroup$ – Gary Walker Nov 28 '17 at 17:59
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    $\begingroup$ Why do you want to know this answer? This may be one of those quirky inbetween questions where the best answer is not actually objective, but rather subjective based on what you want to do with it. If you're looking to use the term to convey a sensation to a reader, the answer may be one way. If you're looking at what a scientist might claim while developing a taxonomy for this strange planet, the answer may be different. $\endgroup$ – Cort Ammon Nov 28 '17 at 20:06
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    $\begingroup$ It's a supercritical fluid. Supercritical fluids are neither gases nor liquids, they are supercritical fluids. You may call it either an ocean or an atmosphere, depending on your mood. $\endgroup$ – AlexP Nov 28 '17 at 21:40
  • $\begingroup$ There is a particular kind of insect, fairyflies, that are so small that they practically treat air as we would water. $\endgroup$ – Draco18s Nov 29 '17 at 3:16
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The fundamental difference between flying and swimming seems to be the density of the fluid - if you have to expend energy to 'stay up', then your're flying, if you can do nothing and not sink/fall then you're swimming.*

Interestingly the viscosity of the fluid doesn't make that much difference when you're swimming in it(as famously shown on Mythbusters, and slightly more scientifically in the paper described in this Nature article)

Because of the above your creatures would definitely be swimming instead of flying, although the difference between the two is less great than you might think - see this video of penguins 'flying' underwater for example.

*This is of course hugely simplified, but broadly true

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    $\begingroup$ I really enjoy it when a blimp swims over the super bowl. ;) $\endgroup$ – Muuski Nov 28 '17 at 20:06
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    $\begingroup$ @Muuski: As David Attenborough once said: “and now we observe a shoal, swimming in their natural habitat. Their bright metallic skins make an excellently confusing defensive pattern. Of course these are not true blimps, but rather their simpler cousins: helium balloons.” $\endgroup$ – Joe Bloggs Nov 28 '17 at 22:23
  • $\begingroup$ Superman doesn't fly - he swims? $\endgroup$ – Nacht Nov 29 '17 at 23:17
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It’s a weird in-between thing
I think it would be a very different very alien world and probably very dark. I don’t think it should count as swimming or flying, although it would be similar to both and probably more similar to swimming than flying. There are already many similarities between swimming and flying as can be seen if you watch birds such as the penguin underwater so its not too much of a stretch to imagine roughly what it might be like.

Although not primarily carbon dioxide a vaguely similar atmospheric effect might well be seen on Jupiter below the cloud tops. Any descent into the deeper levels of the atmosphere would see the pressure rise continuously until supercritical pressures and densities were reached – there is no solid surface.

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There is mechanical overlap between swimming and flying

Yes in swimming buoyancy plays a major role however lift is still a major aspect. When a whale dives, it's using its forward velocity and the angle of its fins and body to create pressure differences allowing it to change its direction similar to a plane.

The difference is speed and pressure play a critical factor in wing size and shape requirements. This is why birds have big wings and whales have tiny ones.

so to answer your question: your creatures would be somewhere between birds and fish in terms of structural aspects, probably closer to fish.

As for how to describe the actual shapes and locomotion that really depends on how your organisms figure it out.

tangent: I saw a science channel episode once that posed the visual that a man standing on venus or Jupiter could actually fly under his own power given cardboard wings because of the atmospheric density.

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    $\begingroup$ Flying on Venus is easy once you figure how to not be on fire. $\endgroup$ – PTwr Nov 29 '17 at 8:34
  • $\begingroup$ @PTwr Exactly, I intentionally skipped over that bit $\endgroup$ – anon Nov 29 '17 at 14:41
  • $\begingroup$ @PTwr, that page also makes the point that in the upper atmosphere, which is much more earth-like, flying is also very much an option; you just have to protect your vehicle (and yourself) against the acid rain. Ironically, in the upper atmosphere, oxygen is also a lifting gas, so you could literally have a blimp you could live inside. $\endgroup$ – Adam Miller Nov 29 '17 at 16:06
  • $\begingroup$ @AdamMiller you forgot about "constant Category-5-hurricane-level winds", with acid rains, very earth like. $\endgroup$ – PTwr Nov 30 '17 at 9:10
  • $\begingroup$ @PTwr, you need to work on your argument skills. I pointed out that the the upper atmosphere is much more earth-like than the lower atmosphere, which is true, any way you look at it. I acknowledged the acid rain. The wind speed is a non-issue, as long as they're fairly constant; they're not going to present much of an issue as long as you're inside them all the time. Why are you trying to argue this anyhow? $\endgroup$ – Adam Miller Nov 30 '17 at 14:35
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Technically, flying is just swimming when you're more dense than what you're swimming through. So yes?

If you're close to the critical point, you could have some fun with the locomotive mode of the creatures though. CO2 has a critical point at 87.98F and 72.9atm. When a supercritical fluid is near the critical point, minor changes in pressure or temperature can drastically affect its density. You could have everything swim with an internal buoyancy motor, essentially squeezing some of the fluid inside itself to sink, coasting along. On relaxing the density would decrease allowing the creatures fluid bladder to go from sinker to floatation device.

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In addition to penguins water ouzels both walk and fly underwater. (Also called Dipper bird, american dipper)

Hal Clement has a novel, "Close to Critical" that takes place on a planet with an atmosphere close to the critical point. At night it rains. Gas has to lose very little energy as there is only a tiny latent heat effect to turn to a liquid. The drops are huge -- feet across.

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    $\begingroup$ How does this answer the question on whether moving without surface contact in supercritical CO2 would constitute flying or swimming? $\endgroup$ – a CVn Nov 29 '17 at 14:59
  • $\begingroup$ By giving an example of: * How the same apparatus can be used in clearly disparate media. * By giving an interesting example of how critical fluids have been used in other stories. $\endgroup$ – Sherwood Botsford Nov 29 '17 at 23:58

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