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Organisms need oxygen, or at least require an atmosphere. The idea is that life with a fraction of earth's atmosphere would have a hermetic membrane that allows them to maintains the optimal conditions for life (carbon based life). So I was wondering, what would be the most air-tight substance an organism can produce?

The ideal answer should...

  1. Put forward an air-tight substance.
  2. If the substance is inorganic, explain where the organism can ingest it and whether or not it can be processed.
  3. Assume the organism needs to move, therefore the substance should be somewhat malleable. (Even if the 'membrane' is a solid, the joints still need to be air-tight.)
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    $\begingroup$ ??? Human skin is perfectly air-tight. In general, lots of animal membranes are airtight. Research subject: what material was used to make the gas cells of historical airships? (And those gas cells contained hydrogen, which is much harder to contain than ordinary air.) Second research subject: what was used originally used as the pressurized air container in inflatable sportsballs? $\endgroup$
    – AlexP
    Commented Mar 15, 2023 at 21:56
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    $\begingroup$ @AlexP agreed! One can even perform an experiment to verify this: take a breath and hold it! Do you leak? (That said of course you get oxygen and CO2 diffusing across the membranes in your lungs, but they're specifically evolved to do that; I think the point stands.) The question does ask about a planet with less atmosphere, but I think most animal membranes could hold up to less than one atmosphere of relative pressure. $\endgroup$
    – realityChemist
    Commented Mar 15, 2023 at 22:02
  • $\begingroup$ If the question pertains to xenobiology, then the answer is not limited to substances that are already known. $\endgroup$
    – Jani Miettinen
    Commented Mar 16, 2023 at 11:20
  • $\begingroup$ So you're talking about little scuba suits for your organisms. The biggest constraint is not containing the air, it's about keeping the pressure for the life of the organism. Human skin, for example, will fail under such conditions because it will gradually stretch. $\endgroup$
    – Boba Fit
    Commented Mar 16, 2023 at 13:01
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    $\begingroup$ I have a coworker that leaks frequently throughout the day. How do I broach the subject of him being a good counter-example to this Worldbuilding SE comment thread? $\endgroup$
    – John O
    Commented Mar 16, 2023 at 14:37

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The most air-tight substance that an organism can produce is likely a combination of natural polymers, similar to those found in some biological structures. One such substance is chitin, which is a complex carbohydrate found in the exoskeletons of arthropods (like insects and crustaceans) and the cell walls of fungi. Chitin is both strong and flexible, making it an ideal candidate for forming a hermetic membrane.

In addition to chitin, the organism could produce proteins and lipids that help create a more air-tight seal. For example, the organism could secrete a natural "glue" made of proteins and lipids that fill gaps and create a tight seal between the chitin structures. This would be similar to the way some mollusks produce proteins and calcium carbonate to create their shells.

To create an air-tight membrane, the organism would need to combine these substances in a way that creates a layered structure, similar to how layers of keratin and lipids create water-resistant barriers in human skin. This multi-layered structure would make the membrane more effective at preventing gas exchange, while still allowing for flexibility and movement.

By looking at these examples, one can imagine a hypothetical creature that combines these elements to create an air-tight hermetic membrane. It could produce chitin as the primary structural component, with proteins acting as a flexible "glue" to fill gaps and provide additional strength, and lipids creating an effective barrier against gas exchange. This combination would allow the organism to maintain optimal internal conditions even in environments with a low atmospheric pressure.

In summary, an air-tight substance suitable for creating a hermetic membrane in an organism would likely be a combination of chitin, proteins, and lipids. These substances would be layered to form a flexible, strong, and air-tight barrier. The organism would need to ingest and process the necessary building blocks to produce this membrane through its diet.

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    $\begingroup$ I believe that during WW2, the Japanese sent two kinds of balloon to drift across the Pacific in an attempt to demoralise America: munitions suspended from balloons of lacquered paper, and trackers on rubber balloons. Unfortunately, it turned out that the rubber was far more permeable to hydrogen than treated paper. It's reasonable to assume that the same would apply to heavier gasses over an extended period, hence that something similar to impregnated paper would be entirely adequate. $\endgroup$
    – Mark Morgan Lloyd
    Commented Mar 16, 2023 at 7:46
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Air tight is not the problem

The most air tight substance is probably silica, produced by diatoms - it's tough, glass like and could form a very good seal. There's a range of biological substances capable of shielding life from low pressures,from things that surround spores, like chitin, to whatever protects tardigrades from vacuum

However, this isn't going to let life continue. Organisms can survive low pressure, but they tend to do it as spores, or otherwise in hibernation, often drying out completely.

Biological life needs gas exchange, and water. Water might be ok for a time, but you'll need to take in oxygen, and let out CO2, or bring in CO2 if your life is a plant. This is fundamentally incompatible with sealing yourself off from the outside. Plants living in hot places have tricks around this, but still have to have pores to do gas exchange, and will lose water in the process.

The same is true of your organism - it has to have pores, the pores will lose water - you need a fix for this bit.

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    $\begingroup$ This is an excellent point, and it would require that the organism must be large enough to be completely self-sufficient if it is supposed to be alive instead of hibernating - or it should actually be a small ecosystem. The garben-in-a-bottle experiment by David Latimer might provide some kind of pointer to this idea: it has only been watered once, 50 years ago, and the sealed bottle is still thriving. See biologicperformance.com/… $\endgroup$
    – Jani Miettinen
    Commented Mar 16, 2023 at 11:24
  • $\begingroup$ @JaniMiettinen - a really nice idea to look at - I've got this image of floating terrariam organisms with the diatom glass-like shells, like soap bubbles. The problem is that it has to only be one stage of their lifecycle, otherwise they'd have no way of breeding easily - they'd need some way of taking in more stuff. Not insurmountable problems, but harder than just making them airtight, which is comparatively simple. $\endgroup$
    – lupe
    Commented Mar 16, 2023 at 12:34
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    $\begingroup$ IMHO the sensible way to do this is to look at all of the cycles that we know about in Earth's biosphere - the water cycle, the carbon cycle, the nitrogen cycle, etc. - and create a symbiotic system in which all of those cycles are replicated on a smaller scale. $\endgroup$
    – Kevin
    Commented Mar 17, 2023 at 0:13
  • $\begingroup$ Nothing protects tardigrades from vacuum. That's pop science nonsense. They can "survive in vacuum" in the same way that a mummy doesn't immediately evaporate when you shine light on it. Outgassing takes more time than the tests on tardigrades lasted, and they were not "alive" in any meaningful way during those tests anyway. Other than that it was possible to revive them. $\endgroup$
    – user102593
    Commented Mar 17, 2023 at 1:09
  • $\begingroup$ @Marvalice - sorry, a slight oversimplification on my part. I'd argue "revives if you bring it down to room temperature and pressure" is still alive - a dried packet of yeast still contains living cells, they're just in a suspended form. Tardigrades seem to do the same thing in vacuum. I thought they'd been exposed to hard vacuum for a couple of weeks though, am I wrong in remembering this? But. broadly, I agree - the only way stuff at the moment can manage in vacuum is by entering into some sort of spore like phase $\endgroup$
    – lupe
    Commented Mar 17, 2023 at 9:39
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Bladders

In ancient Greece, Rome, and China, bladders, from pigs, goats and other animals were used for ball games. They weren't as airtight as modern materials but constantly inflating them from naturally occurring gases produced by bacteria wouldn't be too hard to arrange.

Animal bladders are made of muscle and connective tissue. Yours could be the same. The muscle would provide a steady internal pressure.

Inflated pig's bladder

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Inflating a pigs bladder enter image description here

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Cheeks

Man with cheeks puffed up with air and comical facial expression

Looks pretty airtight to me!

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    $\begingroup$ I feel very sorry for the poor astronauts holding their breath on Mars... $\endgroup$
    – LiveInAmbeR
    Commented Mar 16, 2023 at 14:26
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Some seaweeds actually have air bladders specifically for floatation: https://sciencing.com/function-air-bladders-seaweed-8003965.html https://cdn.britannica.com/07/155207-050-2EFD049F/Bladder-wrack-brown-alga-coasts-Atlantic-Ocean.jpg If there was sufficient evolutionary pressure for such things there are a vast range of natural organic polymers that could be pressed into use. The scope of chemical possibilities is vast beyond imagining. The final solution would be unlikely to be a single substance, but a complex mixture of polymer layers.

To get an idea of the level of complexity in nature have a long here: http://biochemical-pathways.com/#/map/1 Use the controls bottom right to navigate. This is just illustrative of the sheer scale of what is possible in chemistry.

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