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I am currently building a planet that weighs about 4 earth masses and it's radius is 2.3 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.

The plants float and absorb all their needed nutrients from air and use light for photosynthesis. Why would they want to float you ask? So that they can escape predators on the ground and get more light which is normally taken up by the towering megafauna.

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%

I'm planning on making them float using a sac below the stem filled with neon gas (which is present at about a 5% concentration in the atmosphere). The sink (so they don't go up to space) by letting in regular air into the sac. The only issue is how to remove all the heavier gases present and get a mixture concentrated enough with neon that the plant can float. However, noble gases like neon aren't reactive, so how would the plants concentrate the neon?

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    $\begingroup$ Neon is heavier than ammonia and methane. $\endgroup$
    – g s
    Commented Apr 11 at 15:34

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Do it mechanically

Because Neon is non-reactive, it is very good at passing through membranes that other elements would be stopped by. So, you can have secondary bladders that are slowly pulled open allowing just the Neon to diffuse through thier walls to equalize the pressure, then you squeeze these bladders quickly out into your main float bladder which is impermeable to the Neon to inflate it.

That said, methane is better

Of the non-trace elements in your atmosphere, methane, not neon, has the lowest density. In fact, with all that methane and ammonia, neon would barely be buoyant at all. So, a better solution to floatation would be the methane. Since methane is already a common byproduct of biology, it would be pretty straightforward to design an organism, than can fill a float sack with the stuff. Such an organism would never evolve on Earth because of the risk of explosions, but since you have no atmospheric Oxygen to worry about, methane should be a fairly inert substance on your planet.

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  • $\begingroup$ Another good idea might be to use methane to float normally and neon to sink but not completely go to the ground, creating a two way bladder system which I quite like the idea of. $\endgroup$
    – Neil Iyer
    Commented Apr 11 at 18:30
  • $\begingroup$ That would create a nice use of noble gases and so the bladder would be filled with mostly a mixture of methane and neon to control whether it is floating, rising, or sinking. $\endgroup$
    – Neil Iyer
    Commented Apr 11 at 18:59
  • $\begingroup$ @NeilIyer Unless you have story based motives for wanting Neon floating plants, Nitrogen would be a better sinking gas... or better yet, just vent the Methane and let the planet's normal air in. More energy efficient that way, and evolution does tend to prefer energy efficiency. $\endgroup$
    – Nosajimiki
    Commented Apr 11 at 19:10
  • $\begingroup$ Yeah, then. I'll probably just use normal air. $\endgroup$
    – Neil Iyer
    Commented Apr 11 at 19:14

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