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I'm designing a terrestrial planet with 1.15 times Earth's gravity and about 1% helium in its atmosphere, as well as 1.2% hydrogen sulfide. The rest is very earthlike.

A type of animal that has arisen, a photosynthetic beast known as the yellowjelly. It carries two photopigments, a red one and a green one. I'm wondering though, if they were to evolve to fly without the use of conventional wings, how much would they have to change from their jellyfish like ancestors? could they even evolve such an ability in a reasonable timeframe? would using flattened tentacles as pseudowings help?

They're meant to be as small as possible and I imagine they can store helium in their bodies to make them more or less buoyant.

zackit
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    There are several factors that strongly discourage such a thing. For one, floating/flying through the air will be extremely desiccating. It took quite awhile for our ancestors to crawl up out of the water fully, after all. For another, without an absurd amount of buoyancy (which your premise disallows), the shape/configuration is untenable. It just can't provide much lift or steering. – John O Jan 21 '21 at 17:44

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They would need to look like aerostatic balloons.

As I sketched out in this answer the other day, to lift 100 kg one would need above 84 $m^3$ of volume, more realistically around the 150 $m^3$.

Let's look at the best case scenario, where the volume in the cloth is completely devoided of air, the cloth doesn't let any air leak in and can withstand the outer pressure. This means that a cubic meter of that void will have a lifting force equivalent to the weight of the displaced air, which means about 12 N. This means that to lift a 100 kg load would need a volume of about $1000/12 \approx 84 \ m^3$.

That would be a sphere of diameter 6.6 meters. For comparison an African elephant is about 4 meters tall.

They would need to become large spheroids with a membranous photosynthetic skin and would need to be able to isolate lifting gas (most likely hydrogen) inside their body.

It would be interesting to see them floating inside a thunderstorm. 150 $m^3$ of hydrogen struck by a lightning would be quite a boom.

L.Dutch
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    would that make them resemble sort fusions of hot air balloons and hydrogen blimps? but with tentacles and finlike structures? interesting, though i'm wondering if helium could be used too to make it safer from storms. – zackit Jan 21 '21 at 18:35
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    @zackit, hydrogen can be produced from water. Helium has no compounds from which it can be separated. – L.Dutch Jan 21 '21 at 18:51
  • the planet already has alot of helium in the atmosphere being released from vents that the beasts could lay their eggs in to give them a good start on a helium supply i think – zackit Jan 21 '21 at 18:53
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Okay, more gravity would increase the atmospheric pressure. That would mean that at the surface, p=1.15 atm, without getting into the atmospheric composition. The mean molar mass of air on Earth is 28.97 g/mol, helium has a molar mass of 4 g/mol, and hydrogen sulfide has a molar mass of 34.10 g/mol. That means that the mean molar mass of the air is 28.78 g/mol, so the surface-level atmospheric density should be 1.40 kg/m^3. As for how large your airborne cnidarians are, there is a wide variety of sizes of Earthen jellyfish, but let's assume that one weighs 3 kg, and is .16 m wide and .07 m tall. That puts its volume at, assuming an oblate hemispheroid shape, that would put the volume at 3.76*10^-3 m^3. We need a force of buoyancy that equals 33.8 N.

The jelly's density would have to be below 1.40 kg/m^3, including gases within, if it wants to take off. Helium gas should have an average density of .195 kg/m^3, and its maximum density is 798 kg/m^3, which is lighter than water. If we assume that 99% of its volume is taken by helium, then we have a density of 8.17 kg/m^3. But if we assume that it's 99.9% helium per volume, then its density is .992 kg/m^3, which will let it fly. Now, we need to factor in scale height. Scale height on Earth is 8.5 km, and your planet's scale height should be 7.4 km, so the air will thin out faster, but that should give a range of heights these jellies can reach that's a few kilometers.

TysonDennis
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  • the more accurate measurements for my seajelly like organisms are 65 grams, 14 cm wide, and 5 cm tall, and only about 1.3% meat by mass, so how would that change the output? – zackit Jan 22 '21 at 17:51
  • the actual measurements for my seajelly like beasts are about 65 grams, 14 cm wide, 5 cm tall, and 5% of them is meat by volume, the rest being about 65% helium and 30% hydrogen – zackit Jan 22 '21 at 17:55
  • Okay. You should've specified more. The hydrogen should reduce the density, but make them vulnerable to combusting. – TysonDennis Jan 22 '21 at 19:38