I'm currently building a planet, an ammonia world with an average temperature of 225ºK and has a mass of 2 earth masses and a radius of 1.25 earth radii. The average pressure at sea level is 4 atmospheres, and the surface gravity is ~1.28 g.

On this planet, I have a type of chemosynthetic 'plant' that creates large fruit on top of its trunk, essentially looking like a large lollipop. The fruit essentially is a thin layer of material over a pyrophoric liquid filled with spores. When something punctures the thin membrane, the spores are spread out in all directions. The parent may die or be severely injured in the process but at least it released thousands of spores into the world.

The tree needs to be 200 feet tall (at least) although a few feet shorter should be fine. The fruit should be about 20 feet in radius, and so the weight should be a couple hundred pounds (though I may be wrong).

However, the large gravity on this world seems to make this structure unstable, so how can I modify the plant's (or fruit's) structure to hold up the fruit better? Please do not modify the atmospheric composition, pressure, or any other variables; only modify the structure of the plant and/or fruit.

  • $\begingroup$ why do you think it is unstable? $\endgroup$
    – John
    Commented May 1 at 0:27
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    $\begingroup$ A sphere with a radius of 6m (20') will enclose a volume of roughly 900 cubic metres. If the liquid is comparable to water in density then that's around 900 metric tons, not a "couple of hundred pounds". So, do you want a sphere that will store a third of a standard Olympic swimming pool worth of liquid - which would require much more than a "thin membrane" to contain it, or something that will enclose a few hundred pounds? $\endgroup$ Commented May 1 at 3:46
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    $\begingroup$ More to the point, a 20-foot-radius sphere would enclose a colossal number of plant spores, far more than any one plant could possibly need to release at once. I would find it far more likely that the fruit is hollow, basically a balloon covered with a thin layer of spores that can expand to catch the wind when the time is right for dispersal. $\endgroup$
    – Cadence
    Commented May 1 at 5:44
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    $\begingroup$ @Fallenspacerock 4 atmospheres of pressure isn't exactly "deep sea"... you'll find that at ~30m depth on Earth. Humans can swim down that far without specialist equipment (though a bit of practise is required, especially for those wishing to swim back up). $\endgroup$ Commented May 1 at 9:29
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    $\begingroup$ The 'few hundred pounds' estimate is off by about three orders of magnitude, given the stipulation that it's filled with a liquid and solid mixture. If we fill a 20ft radius sphere with supercooled (~25K) liquid hydrogen (weight about 7% of water), it'll weigh 187000 pounds. Subtract about 10000 pounds for buoyancy, the weight of the displaced air assuming a similar mass to pressure ratio as Earth. So 177000 pounds as an absolute lower bound. It's probably not much heavier than water, so about 3 million pounds as an upper bound. $\endgroup$
    – g s
    Commented May 1 at 16:21

3 Answers 3


Heated hydrogen gas

Your bulb has a major flaw. If it pops, it'll not automatically vent it's seeds to the environment. Think of a balloon filled to bursting with marbles. Sure it'll burst, but none of the marbles will fly very far. The pressure itself isn't enough to fly far away. It needs to store more energy. You could have the seeds store some energy by being a bit springy, becoming their normal shape or even unfolding some gliding mechanism if released, but this will not solve your tree structure/weight problem.

To solve your tree problem, we won't solve the tree structure, but the weight. A hot air balloon works in a very simple principle. Air is a certain density per m². If you have something less dense, it'll 'float' up. Hot air is slightly less dense than colder air, so if you get a very, very big bag of air and heat that up, eventually your balloon as well as the gondola below it are together lighter than air. The same is true for helium airships. The full contraption is lighter than the surrounding air. Until you get to a certain height, where the air is less dense.

Mix these together and you can fill the seed with heated hydrogen. The hydrogen is optional though. Instead of filling the whole thing with seeds, you'll only line the edge with them. The rest is heated gas under pressure. This will relatively reduce the weight, as well as propell many of the seeds at high speeds as they are ripped from the seed lining during a burst. This way the weight will be manageable. It also adds an extra danger. These things can not just rupture. They can explode. This might be part of a defence strategy.

  • $\begingroup$ Obtaining helium is very non-trivial. Conversely, hydrogen is fairly straightforward to produce from ammonia, and as an ammonia world probably wouldn't have much free oxygen it isn't so hazardous to work with. Confinement might be difficult, but not nearly so difficult as conjuring up hundreds of cubic meters of helium for every fruit. $\endgroup$ Commented May 1 at 9:24
  • $\begingroup$ @StarfishPrime thanks. I've added that it is optional, and changed it to hydrogen. $\endgroup$
    – Trioxidane
    Commented May 1 at 10:36
  • $\begingroup$ Under the heat of the summer sun, the fruit on top would warm up and eventually the internal pressure would rupture the membrane, and thus it has a natural reproductive cycle. You can add some not-yet-ripe fruit protected from the sun just under the canopy to maintain buoyancy OR bursting causes the tree to collapse and the next generation is now the tallest in the sun. $\endgroup$ Commented May 1 at 20:07

Its an active structure.. as in a biological version of a space-fountain.


Some high-powered underground organs constantly pump in juice at speed (geysirs do that), that keeps the structure upright. Unlike a zeppelin (which is basically a gas-bladder swimming on heavier gas, loaded down with additional weight), this one even survives puncture- for a while, until something frays the arterial system transporting the mass.

Oh and a explosive decomposition would distribute the spores wider into the surroundings. So at least a small over-pressure would not go amiss - oh right, the whole structure houses basically a glorified hotwater/steampipe add some chemicals and voila.



So full disclosure this idea is stolen directly from Steven Baxter & Terry Pratchett's "The Long Mars" and "The Long Cosmos" (the whole series is GREAT for worldbuilding ideas!) On one variant of earth there are ultra-tall trees that use electrolysis to generate hydrogen. The hydrogen in their trees is stored in the wood to permit their massive growth and used to carry water the tops of the miles-high trees. Since your trees aren't NEAR so big you could easily use the same basic principle in a less all-encompassing way to help your "fruit" both to prevent the tree from being too heavy to work and seed distribution. I'm no arborist (much less a chemist) so I can't relay all the details and the wiki itself is a little light on the details compared to the books themselves. But the basic concept is the root system cracks the hydrogen from H2O, and then the plant uses it to make the wood lighter (enabling growth) and when the trees inevitably kinda/sorta explode from a lightning strike or fire (what with all the hydrogen) their seeds are scattered VERY widely into an area partially denuded of competition because the fire has scoured away most of the competing plant life.


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