I am currently building a planet that weighs about 4 earth masses and it's radius is 2.3 times larger than that of earth's. It is an ammonia planet (as the title suggests), and has ammonia oceans with some dissolved water ice, methanol, hydrazine, chlorethane, and salts (sodium, chlorides, sulfates, potassium etc.). The average temperature is about 225ºK on the planet and the avg pressure at sea level is about 4 atm.

This previous question of mine summarizes how these plants work pretty well, but if you don't want to read that question, here is a brief description. These plants float to get more light to power an anaerobic equivalent of photosynthesis that would normally be blocked by megafauna and to avoid predators on the ground.

Here is the atmosphere composition:

  • Nitrogen - 65%
  • Ammonia - 19.7%
  • Methane - 7%
  • Neon - 5%
  • Argon - 1.1%
  • Ethylene - 1%
  • Hydrogen - 0.9%
  • Propylene - 0.2%
  • Other - 0.1%

Currently, the plants have a sac that uses methane produced by their metabolism that helps them rise. They sink by letting in regular air using a series of stomata. One issue I thought of is how they would be constantly sinking and rising, and wouldn't ever "hover" in a small series of altitudes or in one place. This process of constantly opening and closing stomata will also be energy-costly, so I'm pretty sure plants will try to find an alternative.

My question is that is there any way to let the plants "hover" in one altitude or a series of small altitudes so they can conserve energy? Feel free to adjust these plants design in any way to achieve your goals, and its fine if this isn't possible at all.

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    $\begingroup$ I'm not sure I follow - why would they need to constantly adjust their gas bladders? Assuming they're free to more or less drift on the wind, they should be fine with only occasional adjustments. $\endgroup$
    – Cadence
    Commented Apr 12 at 15:18
  • $\begingroup$ Because otherwise they would be constantly floating or sinking and they need to be in a specific range of altitudes. $\endgroup$
    – Neil Iyer
    Commented Apr 12 at 17:09
  • $\begingroup$ @Neil Iyer 2 ways we could probably do this. 1. They can rest at a certain altitude where they don't rise any higher. 2. They release small amounts of gas to keep afloat bobbing up and down with minimal effort ofcourse they have to be very light to do this. $\endgroup$ Commented Apr 12 at 20:29
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    $\begingroup$ @Fallenspacerock Yeah, they have evolved to be as light as possible. I like the idea of releasing small amounts of gas though. $\endgroup$
    – Neil Iyer
    Commented Apr 12 at 20:46
  • $\begingroup$ @Neil Iyer consider that depending on how much gas they have within them there is always an equilibrium in the atmosphere where they hover. You can bopp up and down near this the most easily but it may be easier for the plants to just create more of the light gas to artificially reach the equilibrium wherever they want. $\endgroup$ Commented Apr 12 at 20:52

4 Answers 4


Buoyancy self-regulation via bladder membrane expansion and contraction caused by pressure change

When the the plant goes higher, the pressure drops. As a result, the bladders expand (just like helium balloons when they escape and rise). This expansion pulls increasing number of the pores open that let buoyant gas out, and the plant goes down. As it goes down and pressure increases, the bladders contract and the pores start closing, letting less and less methane out.

Evolution has perfected bladder elasticity, pore behavior and methane production balance in such a way that the plant can maintain desired altitude only using this completely passive method.


Simple dragging ballast tethers: limp cords of vines. The plants roughly adjust their buoyancy so the ends of the tethers lay on the ground or in the water beneath.

Once stable with a length of tether in ground contact, a small increase in buoyancy lifts some of the tether off the ground until the additional weight stops the ascent. Correspondingly, a small decrease in buoyancy leaves more of the tether resting on the ground with the plant reaching equilibrium at a lower altitude. The amount of tether in contact with the ground controls slower biological processes producing and consuming/venting lift gases to prevent wider variations.

If an animal disturbs the tether in an attempt to drag the plant down, it breaks, and the now-more-buoyant plant drifts off while regrowing the tether and venting lift gas to "land" in a hopefully safer location.

  • $\begingroup$ Smart suggestion, but I imagine that would severely limit the height at which the plant can float. Maybe in forested areas the vine lays along the tree canopy, to ensure the plant still retains its primary advantage. I could imagine it as a separate species in the floating plant genus, so OP can still have their 'true floaters' in addition to the 'low floaters'. $\endgroup$
    – M S
    Commented Apr 13 at 11:02
  • $\begingroup$ @MS there would be a limit on how long a tether could be and still support itself, and how long of a tether the buoyancy could support. This could be extended if most of the tether is very thin, only the part that normally lays on the ground needs to be heavy. I don't think they'd have a problem reaching well above the canopy, considering how long real climbing vines can get. And like I said, they may break free at times...if an animal tries to eat them, if they're not getting the sun and nutrients they need, to escape disease or other threats, or just to propagate. $\endgroup$ Commented Apr 13 at 14:28
  • $\begingroup$ @Christopher James Huff and @M S, I like the idea of two different floating plant species, one with tethers and one without. $\endgroup$
    – Neil Iyer
    Commented Apr 13 at 15:22
  • $\begingroup$ @Christopher James Huff Another idea to prevent animals from eating the tether is to add a toxin in it and make it colored bright yellow-green to blue-green (the color of danger in this world). $\endgroup$
    – Neil Iyer
    Commented Apr 13 at 15:40

The Simple Solution:

Your plant could simply keep a small number of stomata open, such that the methane escape rate is similar or equal to the methane production rate. This won't account for variations in metabolic activity, but it should be close enough to be practical.

A Frame Challenge:

The energy costs of opening and closing stomata are absolutely miniscule, and shouldn't be worried about. As such, to add on to my last point, the plant should also be able to precisely control the number of open stomata, to account for variations in metabolic activity.

Another Frame Challenge:

Your plants don't need to hover at a precise altitude, and as such it's unlikely for your plant to evolve such mechanisms in the first place. The minor variations in air pressure and solar radiation that come with altitude changes simply aren't anything to worry about. Unless of course, your world has something specific to it that makes staying within a specific altitude range important.

  • $\begingroup$ @M S No the methane production problem isn't really an issue, because the plants do produce methane in their version of photosynthesis and also, all their nutrients are in the atmosphere. If your curious, they produce energy by synthesizing hydrazine and methane from ethylene and ammonia, and then digesting those two into hydrogen gas and cyanogen. $\endgroup$
    – Neil Iyer
    Commented Apr 13 at 2:11
  • $\begingroup$ @NeilIyer see my edit $\endgroup$
    – M S
    Commented Apr 13 at 2:17
  • $\begingroup$ @M S Yes I see it now. Great answer but in your second frame challenge, the only reason to not go too high is as you go higher, the atmosphere becomes thinner and then the plant dies because it can't get enough ethylene and ammonia molecules, but it's unlikely it would go that high unless something goes very wrong. $\endgroup$
    – Neil Iyer
    Commented Apr 13 at 2:20
  • $\begingroup$ Thinner atmosphere means less buoyancy at higher altitudes (since you're displacing less mass of atmosphere with the same volume), which means the plants will tend to find a level and stick to it. $\endgroup$
    – Cadence
    Commented Apr 13 at 4:51
  • $\begingroup$ @NeilIyer I've done a little bit of research for floating and low-pressure plants (you can see the question I asked about it in my profile), and I think the most dangerous thing about high altitudes would be the low air temperature, not the reduced air pressure. But, with my first suggestion, the amount of methane in your plant's bladder should remain pretty constant, so the only reason your plants should ever get too high up is a physical defect (whether from damage or a mutation) or updrafts from storms and the like. $\endgroup$
    – M S
    Commented Apr 13 at 10:54


  • It's just a ballon:
    Consider your plant to just be a ballon that is lighter than air. Because that's what your plant is so it's safe to assume it behaves exactly the same way.

  • Reaching equilibrium:
    Just like a ballon your plant will eventually rise to a point the air gets thin enough it can't rise any higher. This is what i call equilibrium. Think of it like a boat on water to light to sink but to heavy to start to suddenly fly.

  • Creating your own equilibrium:
    Additionally the plant create this equilibrium almost wherever it wants. Ot can do this simple adjusting the amount of gas in it's bladder to artificially create its own equilibrium. Basically adjusting it self to the air pressure around it to hover.

Adjusting for potential difficulties:

  • Disruption through Wind:
    Due to wind your plant might be involuntarily moved from it's position. To counteract this your plant might be able to release small amounts of gas to stay in place. It would essentially function like what astronauts already use. Though a strong enough wind might require to much gas to counteract while at the same time keep the same altitude.

  • Change in air pressure:
    Would probably happen slowly over time but might change your plants equilibrium. For countermeasures your plant should adjust its gas bladder volume accordingly to the change in air pressure. I.A produces more gas or release some to the keep the same altitude.

  • Sinking air pressure:
    When your tree moves to higher altitudes the air pressure around will sink. This will cause the gas in its bladder to expand and may end up popping your plant like a ballon. But prevent this your plant needs to have enough room in it's bladder to have room for the increased gas volume. Or your plants bladder is strong enough to withstand the increased pressure.

Note: While i am fairly confident on this one there might be still some mistakes. So please correct me if i got anything wrong.


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