Help with Space Setting [SF]

[whswhs]

Originally Posted by myrmidon

Thank you all for the guidance.

Autotrophs would reduce CO to produce methane, as CO2 is solid at the temperatures the planet will be to maintain liquid ammonia. Then Heterotrophs would breathe in methane and exhale CO? H2S is a liquid, though close to its boiling point, in the temperature ranges for liquid ammonia, and SO2 and hydrazine are solids, or close to that temperature. That means an atmosphere of nitrogen as the inert gas, some methane, and trace amounts of CO.

Are you sure about that? Wikipedia says that ammonia melts at -78°C and boils at -33°C, and that carbon dioxide sublimates at -78°C at 1 atmosphere. That sounds like you would have liquid ammonia and gaseous carbon dioxide co-existing.

On the other hand, if you use hydrogen to reduce carbon dioxide, you're going to have

CO2 + 2 H2 => CH4 + O2

and then your planet has an oxygen atmosphere. (Unless the oxygen reacts with the ammonia to form nitrogen oxides, but that would also change the planetary chemistry.)

I'm wondering if you might have reactions involving the formation of hydrogen cyanide, HCN, or of dicyanogen, NCCN? Nitrogen forms fairly stable bonds with carbon, but cyanogen can act as an oxidizing agent; in fact the reason it's so poisonous is that it bonds to proteins that normally react with oxygen, and is really hard to dislodge. Is it conceivable that a planet might have a cyanogen-based respiratory cycle?

[Ulzgoroth]

Originally Posted by myrmidon

Autotrophs would reduce CO to produce methane, as CO2 is solid at the temperatures the planet will be to maintain liquid ammonia. Then Heterotrophs would breathe in methane and exhale CO? H2S is a liquid, though close to its boiling point, in the temperature ranges for liquid ammonia, and SO2 and hydrazine are solids, or close to that temperature. That means an atmosphere of nitrogen as the inert gas, some methane, and trace amounts of CO.

There's some (chemical) elements in this cycle that seem like they could use attention. Reducing CO to methane leaves O that has to be going somewhere. If CO is analogous to CO2 in their exhalation, the oxygen has to be going into whatever the major energy storage molecule would be (the sugar analogue) - which is presumably an oxidizer. That adds up fairly well. But where are the autotrophs getting all the hydrogen?

Also, unlike CO2, CO is quite reactive. It might make sense for animals to discard the incompletely-oxidized carbon, since reduced carbon isn't scarce for them, but the biology to get that out of their relatively oxidizing interior biochemical environment without it reacting with something they don't want it to seems like an issue.

[lwcamp]

If you have oxygen in the form of CO or CO2, and you have hydrogen in the form of CH4 and NH3, then you will get water as well. With water present, you will get a dissolved mixture of ammonia and water, rather than pure ammonia. A consequence of this is that your melting point is suppressed and the boiling point of the liquid is increased. In other words, the range over which your liquid solvent stays liquid increases.

Luke

[myrmidon]

After reading through everyone's comments and doing some further reading, I came up with the following reactions:

Autotrophs (reducing):
CO2 + NH3 + (Light?) Energy --> CH4 + N-based energy compound

Heterotrophs (oxidizing:
CH4 + N-based energy compound --> CO2 + NH3 + Chemical Energy

For the nitrogen-based energy compound, it could be a nitrogen-based sugar, or nitrates. I know nitrates are explosive, especially ammonium nitrates, so that is probably ruled out. Could there be a sugar based on carbon and hydrogen and nitrogen, instead of carbon and hydrogen and oxygen? Or are those explosive as well (I'm thinking of you, TNT)?

I may hand-wave it at this point and say nitrogen-based sugar equivalents, but I'd like to make it work if at all possible.

[Ulzgoroth]

Originally Posted by myrmidon

After reading through everyone's comments and doing some further reading, I came up with the following reactions:

Autotrophs (reducing):
CO2 + NH3 + (Light?) Energy --> CH4 + N-based energy compound

Heterotrophs (oxidizing:
CH4 + N-based energy compound --> CO2 + NH3 + Chemical Energy

For the nitrogen-based energy compound, it could be a nitrogen-based sugar, or nitrates. I know nitrates are explosive, especially ammonium nitrates, so that is probably ruled out. Could there be a sugar based on carbon and hydrogen and nitrogen, instead of carbon and hydrogen and oxygen? Or are those explosive as well (I'm thinking of you, TNT)?

I may hand-wave it at this point and say nitrogen-based sugar equivalents, but I'd like to make it work if at all possible.

For the chemistry you're stating to balance, the energy compound has to contain both nitrogen and oxygen - in fact, it needs to contain significantly more oxygen than nitrogen, since you require 2 oxygen and 4/3s of a nitrogen per methane.

Nitro groups seem perfectly appropriate. They're very rare in Earth biology, but that's no obstacle here. And while you likely would pile them up enough for using an explosive compound to be an issue in energy storage situations (which it isn't at low concentrations in solution), that's not so much a problem unless the rest of the chemistry of the molecule is reducing. Go heavy on nitrogen and oxygen, light on carbon and hydrogen, and you can probably get away with it.

You're probably going to want something bigger than just nitrate, though. It's a small molecule that wouldn't be conducive to building energy-storage molecules, and might be a problem to contain.

Bonus edit: I'm not certain on this, but you probably don't want to have N-N bonds in your molecule, if you're going so far as to plan one out. I think those would be excessively prone to declare their independence as free dinitrogen molecules at the slightest excuse. Low temperatures may mitigate that somewhat, but even so...

[myrmidon]

Originally Posted by Ulzgoroth

For the chemistry you're stating to balance, the energy compound has to contain both nitrogen and oxygen - in fact, it needs to contain significantly more oxygen than nitrogen, since you require 2 oxygen and 4/3s of a nitrogen per methane.

Nitro groups seem perfectly appropriate. They're very rare in Earth biology, but that's no obstacle here. And while you likely would pile them up enough for using an explosive compound to be an issue in energy storage situations (which it isn't at low concentrations in solution), that's not so much a problem unless the rest of the chemistry of the molecule is reducing. Go heavy on nitrogen and oxygen, light on carbon and hydrogen, and you can probably get away with it.

You're probably going to want something bigger than just nitrate, though. It's a small molecule that wouldn't be conducive to building energy-storage molecules, and might be a problem to contain.

That makes sense. I'll just hand-wave it as a sugar analog composed primary of oxygen and nitrogen (mostly in nitro groups).

Thank you all for your help in this discussion, especially Ulzgoroth, whswhs, and lwcamp. I had felt daunted by it for a while, and it is good to work through it with your help. You all rarely disappoint!

[AlexanderHowl]

Sugar compounds do not normally contain nitrogen, so you would be looking at proteins instead (or possibly lipids), unless you wanted to use amino sugar like sialic acid.

[Ulzgoroth]

Originally Posted by AlexanderHowl

Sugar compounds do not normally contain nitrogen, so you would be looking at proteins instead (or possibly lipids), unless you wanted to use amino sugar like sialic acid.

We're not talking about remotely Terran biochemistry here. None of the compounds you're referencing have remotely the right properties.

I'm pretty sure 'sugar analogue' here is mainly referring to biological niche and maybe approximate molecular size. The actual composition is going to use carbon and hydrogen very sparingly. I think a backbone for it would need to feature a lot of N-O-N sequences to avoid both carbon and N-N bonds.