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This question is connected to: Fire Resistant Fauna

I've been pondering a short story concept where a scientific expedition discovered a planet with a huge amount of both free hydrogen and oxygen in the atmosphere.

This is caused by an unobtanium crystal widespread on the surface of the planet which acts as a catalyst, splitting water into hydrogen and oxygen and absorbing heat and sunlight to do so. (That heat then gets released back into the environment when the gases burn).

The resulting atmosphere is highly flammable and as a result flash-fires are a common occurrence. These involve a burst of heat and flame and even mild explosions but are also over very quickly. Very fast-moving flame fronts sweep through the atmosphere, averaging at least one a day, but passing in a matter of moments. Water falls out of the sky in the aftermath of the flash fire and is then split back into oxygen and hydrogen once more by the unobtanium crystals.

Assuming plant life has evolved to survive in these conditions what adaptions could they make to survive? On earth plants tend to either have seeds that survive or allow leaves to burn away and then sprout again. Neither tactic would be effective against near-daily fires though.

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If the flashfire moves very fast, it probably will lack the ability to truly ignite anything. Depending on the heat, anyway. We had an experiment in chemistry class where a balloon full of hydrogen was ignited. Even at short range, it didn't ignite anything and could be safely done against the ceiling tiles and with peope in the room.

Most likely, the flora would not adapt to be fire resistant at all; it would evolve to be heat-blast resistant. Normally I'd say a thin layer of water would suffice, but since the unobtanium crystals would drain it, it'd probably be some other coolant-like substance that can absorb the incoming heat.

The best kind of plants might even use the captured heat for some kind of process. The most logical thing would be to use the captured energy to unfurl the plant's leaves to catch the falling rain.

That would make most of the flora be dormant for much of the day, but when the flashfire comes, they all open up their leaves to catch the falling water. Which would probably be used in part to create a new layer of coolant-material.

(Honestly I think the plant life should be more scared of the unobtanium crystals that will tear the water inside of them apart over the blast of heat)

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  • $\begingroup$ Besides just not growing near these crystals, is there another adaptation these plants could have to let them live with the threat these crystals cause? $\endgroup$
    – Flotolk
    Commented Mar 2, 2015 at 19:39
  • $\begingroup$ That's a hard question to answer, without knowledge of how such crystals work. Which, considering they are a plot-device, is probably "mysteriously". So they could have one, of course, but it would have to be addressed in the story in some way. $\endgroup$
    – Erik
    Commented Mar 2, 2015 at 19:51
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    $\begingroup$ @Flotolk If the crystals work just as a catalyst (meaning any water that touches them gets split, but not actively drawing the water toward itself) then one adaptation would be that if a plant is touching a crystal, the cells in direct contact could dry and harden, becoming impermeable to water, while the rest of the plant is unaffected. Another possibility is that the cells would allow water in, but not out. Earth plants allow water to evaporate out, but these alien plants wouldn't have to work the same way. $\endgroup$
    – AndyD273
    Commented Mar 2, 2015 at 22:12
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    $\begingroup$ @Flotolk Here's another interesting idea for an alien plant adaptation... Since the crystals catalyze water to hydrogen and oxygen, why not give the plants a way to create the water they need by combining oxygen and hydrogen back into water, maybe by using focused sunlight or the heat of the daily atmosphere burning. Plants are just tiny chemical factories after all, creating all kinds of useful compounds out of water, sunlight, and minerals they steal from the soil. $\endgroup$
    – AndyD273
    Commented Mar 3, 2015 at 14:38
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In a flash burn, assuming that the heat isn't super intense most of the plants (and animals) would have some kind of heat reflective coating. Most of the IR band would be reflected. Humans can take some pretty large fire balls without burning up, just loosing hair and low burns. If it's a short lived burn then the protection doesn't need to be that much.

I think going with dessert flora might be a place to start. Cactus have spines ,partly to keep away hungry animals but also the help shade themselves while at the same time have a small surface area to lose water and be subjected to the heat of the sun.

So I think a lot of plants would have a lot of spines. I would also guess that plants with leaves would be much thicker, more like leather to be more heat resistant for short periods of time.

Last some plants will have reactions like the Venus Fly trap, only faster, where they close up at the first hint of a fire storm, kind of like anemones.

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  • $\begingroup$ Thin thorns burn very easily. Other defensive mechanisms are necessary when fire abounds. $\endgroup$
    – Rex Kerr
    Commented Mar 3, 2015 at 10:33
  • $\begingroup$ They only need to resist a few seconds of flame, either bone hard, or 'succulent' (water filled) can easily resist a short burst. $\endgroup$
    – bowlturner
    Commented Mar 3, 2015 at 14:11
  • $\begingroup$ Thorn tips are usually very sharp and will burn off on brief exposure to flame. $\endgroup$
    – Rex Kerr
    Commented Mar 3, 2015 at 19:19
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There are fire resistant plants on Earth that you can model it after.
Redwoods are pretty hard to burn. I recall seeing a video of a guy holding a blowtorch to a piece of redwood, and it not catching. I haven't found the video yet.

https://www.youtube.com/watch?v=BIWc8jntIl0 has a little information on redwoods.

Also: Fire Resistance The high level of tannic acid in the wood and bark in combination with the thickness of the bark (6-12") help to provide a resistance to natural and man-made fires. The growth layer, protected by the thick bark, allow a healing process to occur after damage.

If you want to go with an otherworldly answer, why do your plants have to be carbon based? Silicon based plants would be more fire resistant.
Maybe plants that are carbon based but grow a hardened shell. Diamond is carbon, and you have to get it pretty hot before it'll burn.
How about some kind of slime coating that the plants excrete that acts line a natural fire suppressant.
It really wouldn't take much protection to stop something from burning during a flash flareup like you describe.

If you take a normal sheet of paper, you can hold a match to the center for a few seconds with just a little darkening, but if you hold it to the edge of the paper it'll light right up. So maybe plants that don't have normal leaves where edges can catch easily. Leaves could be thicker and rounded to avoid catching easily.

It seems like high pressure blast waves would be the biggest threat.
http://io9.com/hydrogen-explosion-vs-oxygen-explosion-which-one-wi-1529540558
Is a pretty cool video about burning hydrogen and oxygen versus burning either by itself.
Your plants would most likely be ground hugging or at least very elastic to lessen the chance of being ripped apart by over pressure.
https://www.youtube.com/watch?v=G46oncRAaTQ

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  • $\begingroup$ There is a theory that redwoods (sequoia) are not only fire resistant, but actually depend on forest fires to thrive. Their cones fall unopened and don't allow seeds to sprout. Fire can open cones - which also means that forest floor have been cleared and there is now room for new seedlings to grow. Sadly, it's not much of use in places where fires happen daily, not once per century. $\endgroup$
    – Agent_L
    Commented Mar 3, 2015 at 10:26
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    $\begingroup$ @Agent_L You are correct, and I've definitely heard that before, though being California, it's not once per century, more like once per year. The first Youtube video has a part where the guy is talking about all the undergrowth around the redwoods that normally wouldn't be there because of human intervention in stopping the fires. Almost makes you wonder if humans could make the redwoods go extinct just by fighting all the forest fires... Weird. $\endgroup$
    – AndyD273
    Commented Mar 3, 2015 at 14:21
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The premise is a bit flawed to begin with. Firstly, the hydrogen concentration will have to get above about 0.04 atmospheres (assuming roughly Earth-normal oxygen concentrations); above that it will burn spontaneously. That's not a big problem. What is a big problem is that the combustion will be explosive. If ignition is rare, your plants (and animals) will be torn to pieces by pressure, not heat.

To get an idea of how bad this is, we can take the result that we release about 32 kJ of energy per gram of water created (see here and reverse the process). If the water would have started at 20C, it takes less that 3 kJ to get it to steam, leaving an extra 29 kJ to heat the steam to a toasty 15,000C if you could get it to stay put. Which you can't. From the Ideal Gas Law, pV = nRT and T just went up by 50x while n went down by 2/3 (2H2 + O2 = 2H2O), so your pressure and/or volume must go up by about 30x. Even with a dilute mixture like 4%, this more than doubles atmospheric pressure in a matter of a fraction of a second.

So I'm afraid your plants will get blown to bits.

If we ignore that, then any plant without thin leaves will avoid catching fire. Cacti, moss, succulents, tree trunks, etc., all fail to catch fire easily because of their high moisture content or thickness of their stems. (Tree trunks do not burn easily because you need a high surface area to let off enough gases to combine with oxygen; until your wood is really, really hot (e.g. in a fireplace for a long time), not enough gas gets out to sustain combustion.)

If evolution has been going on for a long time, I'd suppose that most plants would have a clear mucous covering to absorb the flash of heat from the fire. If thick enough, it might even help absorb the mechanical damage of the explosions.

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  • $\begingroup$ I'm a little curious the word spontaneous, as that makes me think of fire without a spark. I found a neat video that shows the difference between pure hydrogen and hydrogen+oxygen mix, and you're correct, the pressure wave would be a huge problem. Two possible solutions: 1. the crystals are in pockets instead of evenly distributed, meaning you'd get pockets/clouds of hydrogen more concentrated in certain areas. 2. The best mix is between 71% and 80% hydrogen. More or less and the combustion slows down a bit. Bonus: Hydrogen rises fast, so a lot of the pressure would be far above the ground. $\endgroup$
    – AndyD273
    Commented Mar 3, 2015 at 14:53
  • $\begingroup$ @AndyD273 - You'll get plenty of mixing before the hydrogen gets very far up. Also, at 71-80% hydrogen, oxygen is limiting. Nothing would be able to breathe after the combustion. $\endgroup$
    – Rex Kerr
    Commented Mar 3, 2015 at 19:16
  • $\begingroup$ That's kind of my point. According to other stuff I've read, hydrogen will ignite in the air at concentrations as low as 10%. The 71%-80% is just the optimal mix if you want to make a really big, really fast bang. At 10% or so you'd get flame, and a smaller bang. Throw other gases into the mix (since a pure hydrogen/oxygen atmosphere probably isn't possible) and the explosion would be destructive but not all consuming... I found a video where trees were observed during a nuclear bomb test, and when the pressure wave hit they bent like crazy, but didn't snap. It's possible. $\endgroup$
    – AndyD273
    Commented Mar 3, 2015 at 19:51
  • $\begingroup$ @AndyD273 - Getting blasted from different directions and intensities on a daily basis is different than a one-off explosion that looks impressive. Note for instance that even strong winds will greatly stunt trees and prevent large ones from growing; it doesn't take that much walking on the ground to kill all the plants there, even really hardy stuff like grass. (And walking applies well under 1 atm pressure, though admittedly grass has no trouble with one step per day.) $\endgroup$
    – Rex Kerr
    Commented Mar 3, 2015 at 22:08
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Lichen and other Resurrection plants are probably the best suited to such an environment, specifically the lack of available water and resistance to flash-fires. It's likely that other plants would evolve alongside or parasitising the resurrection plants, perhaps plants coated in lichen, rocks filled with colonies of lichen, patches of moss that release CO2 when the humidity dips below x%, etc.

Though as Erik said, the greater threat would be from unobtanium crystals in sand and dust falling on the plants and destroying their tissues.

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Both your flora and fauna will have to be based on some very exotic biology, because it could not contain any water. Basic chemistry of life would have to be very different, so not sure how you can even define plants. Plants (defined as life form using solar energy) are not necessary at your planet, because crystals of unobtainium make the work plants do on Earth (capture sun energy to and provide sustenance to other life forms).

On such planet, with plentiful free energy from oxidization of hydrogen, there would be very little advantage for slow sedentary lifestyle of plants.

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  • $\begingroup$ So long as it keeps the water away from the crystals I don't see a problem. Water can exist inside plants & animals so long as they keep it inside themselves. $\endgroup$
    – Tim B
    Commented Mar 2, 2015 at 21:03

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