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Old 11-08-2019, 09:01 AM   #1
whswhs
 
Join Date: Jun 2005
Default a science question: atmospheric disequilibrium

GURPS Space lists a variety of planetary types defined in large part by atmosphere. Two of them are Ocean and Garden. An Ocean world has liquid water and a carbon dioxide/nitrogen atmosphere; a Garden world has photosynthesizing organisms that break up water into oxygen and hydrogen, combining the hydrogen with carbon dioxide to produce organic matter. Ocean is near chemical equilibrium; Garden is not.

All the other planetary atmospheres seem to be near chemical equilibrium. I suppose this is natural enough; the ones in our solar system are apparently near chemical equilibrium. But if we're going to suppose that other planetary types can have life, that life ought to be capable of energy capture and of creating disequilibrium states with its waste products, and perhaps closed cycles exploiting those waste products as a fuel.

What could the resulting atmospheres be like? If you start out with, say, a hydrogen based atmosphere, and life forms native to that atmosphere do something analogous to photosynthesis, breaking up an available liquid or gas, what are the waste products, and what kind of closed chemical cycle might they create? Would the atmosphere be as different from a gas giant atmosphere as Garden is from Ocean? What if you start out with, oh, an ammonia world? Are there new possible atmospheric types?
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Old 11-08-2019, 10:17 AM   #2
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Default Re: a science question: atmospheric disequilibrium

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Originally Posted by whswhs View Post
What could the resulting atmospheres be like? If you start out with, say, a hydrogen based atmosphere, and life forms native to that atmosphere do something analogous to photosynthesis, breaking up an available liquid or gas, what are the waste products, and what kind of closed chemical cycle might they create? Would the atmosphere be as different from a gas giant atmosphere as Garden is from Ocean? What if you start out with, oh, an ammonia world? Are there new possible atmospheric types?
The purpose of photosynthesis is after all to extract the stuff you build your body from out of the environment. Since life is going to be made mostly of carbon, the likely atmospheric interactions are inhale CO2 and exhale oxygen or inhale methane (or some other light alkane) and exhale hydrogen (or water vapor if you have an oxygen source). I suppose you might extract nitrogen from ammonia, exhaling hydrogen, or oxygen from water vapor (again exhaling hydrogen), sulfur from H2S (exhaling hydrogen). There aren't really very many plausible atmospheric gases that you could break apart and dump a gas other than oxygen or hydrogen, and hydrogen is probably generally uninteresting - an atmosphere that doesn't already have *lots* of it probably means a world that doesn't have enough gravity to hold onto it long enough to change the atmosphere dramatically.

The reverse, respiration - burning atmospheric gases with something else for energy - might get you a handful of other options - letting you consume ammonia (exhaling hydrogen and nitrogen) or perhaps sulfur oxides exhaling H2S.

Not that *small* amounts of gaseous waste products can't change an atmosphere in ways that matter to human explorers. It doesn't take a lot of HF or HCN or O3 or halogens for humans to decide it's a different atmosphere type, even if it isn't enough for the rocks to care much.
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Old 11-08-2019, 10:31 AM   #3
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Default Re: a science question: atmospheric disequilibrium

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The purpose of photosynthesis is after all to extract the stuff you build your body from out of the environment. Since life is going to be made mostly of carbon, the likely atmospheric interactions are inhale CO2 and exhale oxygen or inhale methane (or some other light alkane) and exhale hydrogen (or water vapor if you have an oxygen source). I suppose you might extract nitrogen from ammonia, exhaling hydrogen, or oxygen from water vapor (again exhaling hydrogen), sulfur from H2S (exhaling hydrogen). There aren't really very many plausible atmospheric gases that you could break apart and dump a gas other than oxygen or hydrogen, and hydrogen is probably generally uninteresting - an atmosphere that doesn't already have *lots* of it probably means a world that doesn't have enough gravity to hold onto it long enough to change the atmosphere dramatically.
The basic reaction on Earth is that you split hydrogen off of water, combine it with carbon dioxide to get carbohydrate and water, and dump the oxygen from the water into the atmosphere. You can do that instead by splitting hydrogen off of hydrogen sulfide. It's not obvious to me (a) what the starting organic molecule is, (b) what element you are adding to it to get organic monomers that can be the basis for life, and (c) what element you're dumping into the environment after you split a source molecule to enable (b), in the various cases you discuss.
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Old 11-08-2019, 10:32 AM   #4
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Default Re: a science question: atmospheric disequilibrium

You need two elements to form the basis of an atmospheric disequilibrium. Earth actually uses three: Oxygen, Carbon, and Hydrogen. Its worth noting that the Carbon and Hydrogen generally become solid or liquid and stay on the surface of the earth, preventing earth from experiencing constant fuel-air explosions.

A hydrogen based atmosphere is going to need an additional element in order to create a disequilibrium. Also, outside of a gas giant, hydrogen atmospheres are tricky to keep bound to the planet.

If Hydrogen and Nitrogen are your elements (ammonia), you're looking at splitting them into diatomic pairs (elemental hydrogen and elemental nitrogen) which both have very low condensation points, unless you've got another element you can mix in.
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Old 11-08-2019, 11:13 AM   #5
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Default Re: a science question: atmospheric disequilibrium

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The purpose of photosynthesis is after all to extract the stuff you build your body from out of the environment.
Maybe to some degree, but mostly it's about using energy from sunlight to pay for reactions that would otherwise have negative enthalpy, and the resulting products are used as fuel. It's not really required that there be any gaseous substances at all.

However, if there are gaseous products, you can assume they're ones that require energy to split off, and there aren't terribly many candidates for that; splitting off light hydrocarbons won't be all that useful, nitrogen tends to be pretty inert once it gets split off so you can't have a long term, fluorine and chlorine aren't common enough.
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Old 11-08-2019, 11:31 AM   #6
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Default Re: a science question: atmospheric disequilibrium

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However, if there are gaseous products, you can assume they're ones that require energy to split off, and there aren't terribly many candidates for that; splitting off light hydrocarbons won't be all that useful, nitrogen tends to be pretty inert once it gets split off so you can't have a long term, fluorine and chlorine aren't common enough.
Conceivably you might split hydrogen cyanide into hydrogen and cyanogen. Cyanogen has quite a low boiling point.

I don't insist on dumping the waste product into the air. If you split hydrogen sulfide and get sulfur, you might have "respiring" organisms seeking out deposits of sulfur to consume, I suppose. Or maybe the sulfur could combine with something other than hydrogen.

I realize that the answer might be "there are no respiratory cycles other than water/oxygen that could sustain a planetary ecology." But I'd like to see the list of planetary types in GURPS Space worked through.
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Old 11-08-2019, 11:57 AM   #7
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Default Re: a science question: atmospheric disequilibrium

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Conceivably you might split hydrogen cyanide into hydrogen and cyanogen. Cyanogen has quite a low boiling point.
Where are you getting significant amounts of hydrogen cyanide from? It has positive enthalpy of formation. I mean, CH4 + NH3 -> HCN + 3H2 might be a useful reaction, or CO2 + NH3 -> HCN + H2O + O, but you'd be keeping the HCN.
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Old 11-08-2019, 12:03 PM   #8
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Default Re: a science question: atmospheric disequilibrium

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Where are you getting significant amounts of hydrogen cyanide from? It has positive enthalpy of formation. I mean, CH4 + NH3 -> HCN + 3H2 might be a useful reaction, or CO2 + NH3 -> HCN + H2O + O, but you'd be keeping the HCN.
That's exactly the kind of discussion I was hoping for.

So okay, is it conceivable to run it backward? Your energy harvesters produce CN, and dump it out in the form of HCN or NCCN, and then other organisms take it in and reverse the reaction. Yes, it's toxic, but then oxygen is toxic too; there are organisms that die on exposure to it.
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Old 11-10-2019, 09:03 PM   #9
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Default Re: a science question: atmospheric disequilibrium

In prep for a previous campaign I looked into what alternatives there might be to hemoglobin. It brought me here: https://worldbuilding.stackexchange....rs/28380#28380 and if you scroll down far enough you'll find reference to the easily pronounceable "Chloro-carbonyl-bis(tri phenylphosphine)-iridium", which would allow the blood to transport both oxygen and hydrogen.

I imagine that this could of course give you a species that lives on a planet with free hydrogen but no free oxygen, though I have no idea how you'd keep the supply of hydrogen in the air. I suppose the plant analogues could breakdown free hydrocarbons (A stand-in for CO2) into carbon and hydrogen, but as far as I know that generally requires oxygen involved in the process somewhere and the result is H2O, not free hydrogen. At which point you'd need to find some way to split the hydrogen and oxygen, which is a process that costs energy, rather than supplying it.

Maybe I should have paid more attention in chemistry class, but in any case, you have an example of a way to metabolize something other than oxygen.
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