This is not a question on whether or not floating, balloon-like organisms are biomechanically viable - I already know the answer to that, which is yes. This instead deals with the plausibility of such a thing evolving, a problem I've thought about for quite some times.

So, in my alien world, there are giant colonial invertebrates, similar to siphonophores, and they form huge balloon-like sacs, floating through the sky and filter-feeding aerial plankton. They float by means of hydrogen gas, which they produce by electrolysis (using biolectricity to split water molecules in the air).

However, I've recently discussed this concept with others, and it seems as though balloon organisms are quite a difficult thing to justify, mainly because of the energy expended in producing the hydrogen gas. A friend did some research, and according to his Google searches, it takes around 4 kilowatt hours to split one litre of water into hydrogen and oxygen gases - giving you 1,200 litres of hydrogen gas. That can lift roughly 4 pounds in an Earth-like atmosphere, giving you the capacity to lift roughly 1 pound of mass per kilowatt hour. 1 kilowatt hour, I'm told, costs 3,500 kilocalories to produce.

Now, before I continue, let me say that my planet does not have conditions identical to Earth. The gravity is about 0.85 times lower, while the atmosphere is denser - perhaps a quarter of the way or a little under between Earth and Venus. Does anyone know if it's feasible to alter the calculations above to apply to these conditions?

So, even in a place with an atmosphere about 20 times denser than Earth's and a gravity 0.85 times lower, I doubt that the conversion rate of 1 Kwh = 1 lb will alter drastically. Sticking with that initial equation, even if the balloon colony was so light that it weighed only as much as a human, it would still expend about 500,000 kilocalories lifting its mass, which, let's face it, is ridiculous - it's 1 million times the daily calories spent on a male human brain.

So, I guess the first thing I need to know is how much will the altered conditions change the 1 Kwh = 1 lb equation? If it does by a lot, which I doubt, then it's a start.

Then there's the question of how can I go on to reduce the tremendous cost in energy of floating with hydrogen? Their lifestyle isn't exactly the worst imaginable for reducing energy expenditure, in fact. Here are a few things which should help by cutting calory intake in other areas of life:

  • Passive filter feeding lifestyle
  • They are colonial and thus do not move
  • Ectothermy
  • Specialized zooids: only special castes of individuals produce hydrogen, the rest perform single tasks e.g. reproduction, digestion, defense

So, here is my main question, summarized:

In an atmosphere 20x denser than Earth's and with gravity 0.85x that of Earth, could organisms which have the traits above float by means of a huge bladder of hydrogen gas, producing the hydrogen via electrolysis? Are there other methods of biological hydrogen production, real or speculative, that are more energy-efficient?

Note: the gas must be hydrogen, and I would prefer if hydrogen production methods which require photosynthesis were not used in answers.

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    $\begingroup$ One BIG problem you'll have is that hydrogen is very hard to contain long-term. Unless I'm mistaken, we've never built a container that can hold hydrogen without leaking. We can get the leak pretty slow, slow enough for zeppelins to use it, but given how many calories it costs to make, I don't know if your balloon creatures could ever afford it. Why not change the atmosphere so you can use a different, easier-to-obtain lifting gas? $\endgroup$
    – Ryan_L
    Commented Oct 27, 2018 at 18:35
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    $\begingroup$ 1. You don't need to use electrolysis. Hydrogen is already a byproduct of many biological processes. Our mitochondria already make hydrogen, they just then combine them with oxygen to make water. If you skip that step you can produce hydrogen as a byproduct of anaerobic respiration, see Hydrogenosomes. 2. You should think in terms of hydrogen production vs. hydrogen leakage instead of quantities. The organism doesn't have to make all its hydrogen in a day. If they grow gradually that cost is spread over a long time. Instead worry about loss. $\endgroup$ Commented Oct 27, 2018 at 18:36
  • $\begingroup$ I like @MikeNichols' idea. We lose cells (including blood) all the time. The evolution of the proposed creature would (a) have a bladder lining that was reasonably well sealed for the purpose and (b) would have a system (perhaps not unlike bone marrow) to replace lost hydrogen on a constant basis. Frankly, your only real problems are lightning and volcanism (or anything that starts a wildfire... bang! rats...). $\endgroup$
    – JBH
    Commented Oct 27, 2018 at 21:10
  • $\begingroup$ Related, but not a duplicate: worldbuilding.stackexchange.com/q/110613/21222 $\endgroup$ Commented Oct 28, 2018 at 17:36

3 Answers 3


A living organism wouldn't produce hydrogen via electrolysis, it would use a chemical reaction (as Mike Nichols points out). In Peter Dickenson's book The Flight of Dragons, he covers several biological processes which already exist in living creatures that produce hydrogen as a byproduct.

Second, if the creature can start very small and grow over a period of many years, the energy required to generate that hydrogen isn't at all unreasonable. A human child will consume something like 1.5 million kcal in order to grow into an adult. So 0.5m kcal isn't a lot over a period of years.

Your bigger problem is keeping these animals from cathing fire or losing all their hydrogen. Again, Dickenson talks about this in his book.


At an atmosphere 20x denser than Earth's and with gravity 0.85x that of Earth, could organisms which have the traits above float by means of a huge bladder of hydrogen gas, producing the hydrogen via electrolysis? Are there other methods of biological hydrogen production, real or speculative, that are more energy-efficient?

Noone on this planet has the expertise to answer the question. Lacking the gravity to retain such a dense atmosphere if it were composed like ours is a huge, deal-breaking issue.

The (rough) mean air density on Earth is 1.2 g/L even if the atmosphere were pure Argon the density would only be 1.78g/L, so let's go denser: Krypton - not dense enough, Xenon - no way - Were it mostly Radon, it would still only be 9.73g/L - still nowhere near enough to supply your 20* atmospheric density. (Oganesson, the next and densest Noble gas is theoretically predicted to be solid at STP.)

If the Question is to be science based then I think that you must do some handwaving regards either the atmospheric composition (invent a new gas) or regarding some kind of containment field for the atmosphere which compresses it to the density you require

Otherwise - you'd need to increase gravity somewhat.

The most probable postulate is a massive not very dense planet - near gas giant size - ie. somewhere between the size of Earth and Neptune - nearer Neptune (look at atmosphere section here plus this for some basic calculations.). Basically, we don't know enough yet to be accurate, you might as well speculate with relative freedom.

The principle is that gravity decreases in an inverse square law with distance from the planet's centre - so if there is .85G at the surface then the larger the radius of the planet the higher you would be able to go above the surface before gravity diminishes sufficiently to not be able to hold onto the atmosphere - the thicker the atmosphere can be and therefore the denser it will be at the surface. TL-DR - a bigger planet is better.

Look to comments for ideas regarding mitochondria and hydrogen production.

Or a simple Aluminium ion V's a Hydroxyl radical reaction is just fine if you wish - there are many plants on earth that accumulate Aluminium and it could form the basis of a defence system like the silica hairs found on stinging nettles - alumina hairs instead.

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    $\begingroup$ "Lacking the gravity to retain such a dense atmosphere if it were composed like ours is a huge, deal-breaking issue." Venus- "Am I a joke to you?" $\endgroup$
    – Alastor
    Commented Nov 25, 2022 at 15:26

The lower gravity is effectively irrelevant. Buoyant forces are independent of gravity. So the only thing that matters is the density of your atmosphere.

The amount of mass that a given volume of hydrogen can lift is equal to the difference in mass between that volume of hydrogen, and the equivalent volume of surrounding air. Since hydrogen is already pretty light, if you double the density of the atmosphere without increasing pressure, the lifting capacity does not actually double, but it comes pretty close! You would do that by, e.g., filling the atmosphere with heavier gasses somehow.

Making your atmosphere CO2-dominated, like Venus or Mars, rather than N2 dominated, will increase the density compared to Earth by a factor of about 1.5, and increase your lifting capacity by a slightly lesser amount.

Another method of increasing density is to simply increase the pressure. That will increase the density of the hydrogen at the same time, but but that actually turns out to make the math simpler--everything cancels out, and it turns out that multiplying the density of gas corresponds to an exactly proportional increase in lifting capacity per volume. So, if you start with a basically-Earthlike atmosphere, and increase the pressure to 20 bars, without changing the molecular composition, you will get 20 times as much lifting capacity out of the same volume of hydrogen. But to get that volume, you need 20 times as much mass in hydrogen... so it's all a wash. Increasing the pressure doesn't really help--except that reducing the required volume reduces the required mass of the container, so it does actually help a little bit. High pressure means smaller, less massive, and more robust creatures can use aerostatic flight.

Another thing to consider is that you can gain lift capacity by heating the hydrogen (reducing it's number density, and therefore its absolute density, below the number density of the surrounding air). A large organism could effectively heat itself with the sun, without having to actually use photosynthesis, rising during the day and descending during the night as it cools down. A smaller organism, such as could exist under 20 atmospheres of pressure, would be less able to use solar heat (although hardly unable), but would be better equipped to insulate itself against loss of heat from the lifting chamber, and perhaps pump waste metabolic heat into its lifting gas as well.


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