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My setting I've been working on for the past few years has giant floating creatures (I know, very original) that the local sapients often live on like sky islands, maybe cultivating some kind of airborne algae on them. With a gravity of 1.18g (or as low as ~0.6g if necessary), how dense would the atmosphere need to be to support a creature big (and, in places, strong) enough to have a small, by our standards, house on their back? Additionally, what reason would such a creature have to be floating around at least a couple hundred metres above sea-level?

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    $\begingroup$ In general it's best to take one question at a time. Your question about the reason to be living up there is worth its own chapter, and could (and likely should) be removed and reserved for later. For this specific reasons question, I invite you to think strategy beforehand : What is up there that you could gain from : food, free space, vantage point...? What is there you don't want to be next to below (generally hazards)? This should give a basic goal to reach, both for you and us if we try to answer this question. $\endgroup$ May 12, 2023 at 15:59

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With a gravity of 1.18g (or as low as ~0.6g if necessary)

Gravity is irrelevant; for aerostatic lift, gravity cancels out, and a given aerostat will have the same mass-lifting capacity in a given atmosphere under any gravitational pull.

how dense would the atmosphere need to be to support a creature big (and, in places, strong) enough to have a small, by our standards, house on their back?

How small of a house? We can build airships that fly in our own atmosphere, and airships are theoretically possible on Mars.

Ultimately, size also doesn't matter for aerostats--as long as you can maintain low enough density, aerostats of equal density will float just as well at any size. Airships are big just because of square-cube scaling laws which make it easier to construct lower-density structures at larger sizes. A lower atmospheric density just means that you need a bigger lifting volume per unit of payload--or, in your case, a larger creature per size of house.

So, decide how big a "small house" is, and how large you want the creatures to be, and that'll tell you how dense the atmosphere has to be for a creature of that size to support a house of that size.

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Fine descriptive answers, but no quantitative answers to an ultimately straightforward physics question, so:

The atmospheric density must be the same as the density of the structure you want to be neutrally buoyant in the atmosphere: $(m_{house}+m_{fish})/(V_{house}+V_{fish})$.

Where $m$ is mass and $V$ is displaced volume. You can probably approximate $V_{house} = 0$ since most of the space it seems to take up is just air. Gravity is irrelevant except as a contributor to atmospheric density.

I think anything you do to try to scientifically justify such a creature will be less plausible than magic, so I would suggest just using magic and calling it a day.

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FRAME CHECK: Cyanobacteria might help solve the problem

Rather than worrying so much about gravity etc there are species of cyanobacteria that produce hydrogen gas as part of their metabolic processes. You could stretch the rules of biology (as understood on Earth) to posit the ancestors of your jelly fish entering a symbiotic relationship with one or more strains of these bacteria.

Initially these bacteria resided in the small transparent hollow spaces in the top of the jelly where they had access to sunlight, protection from the elements and a humid environment. In exchange they produced hydrogen and some of the jellies nutritional requirements. Over time the surface area of the jelly increased to allow more sunlight to reach the bacterial and more food to be produced. And as they got larger the jellies also evolved to trap more of the hydrogen as a weigh saving technique. Slowly it got to the point where, from time to time they would be lofted out of the ocean and into the air during high winds until finally they reached the point where they could float above the ocean's surface more or less permanently.

While this helped protect them from predators the problem remained however that the the bacteria still don't supply the animals with all of their nutritional needs so the jellies have to return to the oceans surface from time to time to feed/hydrate and possibly also reproduce.

Keep the process going long enough and you end up with large, if not giant jellyfish who use multiple eyes to track the sun/moon/stars combined with flexible sails (crests) and/or specialized limbs as 'rudders' to navigate your worlds winds and oceans. You might even imagine annual migrations around the circumference of the planet with the jellies instinctively rising higher in the air as they approach land masses and going into fasting mode until they reach the ocean again. Of course most, if not all of them may fail to make it so every year the new generation is founded by the luckiest if not best 'flyers'.

Hope this helps.

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I don't think that's happening at at the g levels you propose, at least not for a creature of "reasonable size" and/or livable conditions. Atmospheric density is related to various factors such as composition, gravity, and pressure. But changing any of these enough to make things easily float also makes them quite deadly. They don't really stay gaseous as they get denser and denser and become inhospitable long before they ever get that dense.

Let's take a houseboat as an example. That's a floating house. So the atmosphere would be more of an ocean of liquid than a gaseous atmosphere. But atmospheric pressure is a gradient so that means that the altitude where the house could float (i.e. where your head is as you walk around) would be almost just as dense. You'd basically be in an ocean of sorts. Quite inhospitable.

Another example is Venus's atmosphere which is very dense but still gaseous. At 6.5% the density of air at the surface, something made to float it could quite reasonably float though not as easily as on water but much easier than floating in the air on Earth. But that level of density requires pressure and temperatures that are quite inhospitable. You don't want to be walking around in that.

The only way around this would be to go the hot air balloon (or probably helium) approach and have the creature be extremely, extremely, extremely large compared to all the stuff on its back. In that case, nothing special is really needed. You could nudge up the atmospheric density a little bit just to make it so the size of the floating organ doesn't need to be quite so large but it's basically immaterial because it will still be enormous.

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    $\begingroup$ "Atmospheric density is related to gravity" Citation needed. Venus and Titan both have much denser atmospheres than Earth with less gravity. $\endgroup$ May 11, 2023 at 19:20
  • $\begingroup$ @LoganR.Kearsley I did not say it was wholey dependent on. Though, for a liquid atmosphere you probably do need gravity and not pressure and temperature tricks like Venus. I have modified that sentence a bit though. $\endgroup$
    – DKNguyen
    May 11, 2023 at 19:22
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    $\begingroup$ "Atmospheric density is related to gravity and pressure." Only trivially, insofar as greater gravity with identical atmospheric mass will result in increasing pressure. Pressure is a function of gravity and atmospheric mass, and if you choose to vary the pressure, either the atmospheric mass or the gravity must change. The independent variables are pressure vs. composition, or gravity, depth (which is interchangeable with atmospheric mass), and composition. $\endgroup$ May 11, 2023 at 19:27
  • $\begingroup$ How's that modification? My main point is that if you change anything so it easily floats, people start dying. $\endgroup$
    – DKNguyen
    May 11, 2023 at 19:32
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What does the jellyfish use for muscle?

Electrostrictive gels change their size on an electrical stimulus. Some of them reject or absorb water to change their size. However, there is no reason that I know of that prevents you replacing water with air, or even vacuum. You could have an electrostrictive foam that can pack close on one electrical stimulus, squeezing out the air; and re-expand on another stimulus. This would be very delicate, but aerogels can hold their shape and support small objects.

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There are limits to the atmospheric pressure which humans can survive in. If human characters visit that world and don't wear breathing masks or even spacesuits to survive, the atmosphere has to be within those limits.

Humans need between 60 mmHg and 400 mmHg of oxygen to survive, compared to Earth's total atmospheric pressure of 760 mmHg. They also need a little nitrogen for the sake of plants, a little carbon dioxide, and a little water vapor in the atmosphere to survive.

Humans can survive in an atmosphere with several times the pressure at Earth sea level if the oxygen is diluted with non toxic and largely inert gases like nitrogen and the noble gases. Thus a breathable atmosphere could have a total pressure between about 70 mmHg and several thousand mmHg.

See https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf pages 13 to 19.

Of course if the only characters in the story are natives of the world who evolved to breathe the atmosphere they might be able to breathe in a greater atmospheric pressure than humans could survive. But there would still be some upper limit to the pressure they could survive.

I note that humans can built airships that sometimes have outer shells that humans can walk on without plunging through them. Of course humans don't want to fall or be blown off the tops of airships and so only rarely go on top and stay close to the centerline where the upper surface is almost flat. And the centerline might be reinforced to bear human weight more than most of the upper surface of the airship.

I note that the "houses" or gondolas of airships are hung from the bottom of the airships instead of resting on top of them. No doubt structural changes would be needed to build the gondolas on top of airships without their weight causing the airships to turn upside down.

So airship designs should give you some information about how large an airship has to be to carry a gondola of a specific size or weight in Earth's atmosphere. Of course that would have to modified to put the gondola on top.

Natural gas bag creatures floating in the atmosphere of a planet would not usually have evolved to have human sized creatures walking on their upper sides or to have houses built on their upper surfaces. If humans tried to walk or build houses on their upper surfaces, they would probably plunge though the upper surfaces, fall through the gas bags and fall through the lower surfaces to their deaths.

So maybe you can think of a reason why the giant gas bag floating creatures evolved strong but light weight outer skins on their top sides. Possibly the creatures normally carry something on their tops which the people of the world demolish and throw the pieces overboard when building their houses on the top.

Or maybe an advanced civilization long ago designed the floating creatures and used genetic engineering to grow them, and they were designed by that ancient advanced civilization to be able to have people walk on them and to carry houses.

Unless the people of your world have wings and are natural flyers, they would need to invent aircraft to reach the large floating creatures. Or maybe the large floating creatures sometimes land on the ground to lay eggs or something, and the people climb on them when they are on the ground. Or maybe when the large floating creatures float low, the people climb up their dangling tentacles to reach them.

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