Ammonia planets

Question

In an alien atmosphere, where methane is the main gas instead of oxygen and ammonia is the main solvent instead of water. Similar to how photosynthesis on earth plants work, by taking in carbon dioxide and water to create glucose (source of food) and oxygen (waste product). What gas would plants on this planet take in alongside ammonia that would result in the release of methane as a waste product as well as their source of food.

For visual perspective- NH3 + ____ + energy = ____ + CH4 Atmospheric composition if needed: Nitrogen- 93% Methane- 6% Other trace gases- 1%

I hope this is more clear

Answer 1

What plants do on Earth is oxidize (remove hydrogen from) water, and use that hydrogen to reduce carbon dioxide into sugars. That process takes energy. An analog with ammonia and methane would be the following:

2NH3 + C2H4 + Energy -> N2H4 + 2CH4

What is happening is that the ammonia is being oxidized and its hydrogens being transferred to the ethylene, to form Hydrazine and Methane. According to Wolfram Alpha, this requires 83.2 kj/mol, and therefore would occur in a plant. The reverse reaction would release energy, with Hydrazine being used as food.

Answer 2

I'm not a biochemist. Biochemistry is super-duper complicated. The following is essentially wild speculation, but hopefully it can help guide your thinking.

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The overall equation for photosynthesis on earth is:

$$6 CO_2 + 6 H_2O + \gamma \rightarrow C_6H_{12}O_6 + 6O_2$$

Here are the important things to note about this process, from the perspective of changing it. These features need to be present to allow for anything like the photosynthesis we see on modern earth:

1. The reaction requires energy to proceed (is endothermic), rather than releasing energy when it proceeds
2. The solid output of the reaction (glucose) is a store of energy that is stable enough not to spontaneously decompose
   2. (A) Glucose is a moderately complicated molecule. This allows the biochemical system to manipulate it with a good degree of specificity using targeted proteins, and minimizes the chances that it will cause unwanted side reactions.
3. The byproduct of the reaction is a gas, which can easily escape the plant. Liquids are OK too, but not solids (which are difficult to transport out of the plant)

You ask about instead using the reaction scheme:

$$nNH_3 + mX + \gamma \rightarrow iY + jCH_4$$

You also posit that this ammonia exists in a liquid state (analogous to water). This is your first problem: ammonia boils at -33$^{\circ}$C. This is a problem because at this temperature, about 50$^{\circ}$C colder than on earth, and as a rule of thumb every 10$^{\circ}$C difference results in a factor of 2 change in chemical reaction rates. That means that reactions on this planet will take place about 32x slower than on earth, which makes it unlikely that an endothermic reaction like this could take place.

You have a few ways around this. Perhaps your biochemistry includes a ubiquitous reaction that is quite exothermic, which is used to locally heat life enough for reactions to happen at a reasonable pace. Perhaps life only grow around geothermal hot-spots, where the temperature is locally higher and the ammonia is gaseous. Or perhaps the planet has a similar temperature to that of earth, but an atmospheric pressure about 10x higher, allowing liquid ammonia at room temperature (this likely creates its own set of problems).

Anyway, passing that on, let's see if we can come up with a moderately stable, moderately complex nitrogen compound to replace sugar. That will guide the rest of the reaction. I think an amino acid is probably a decent choice. I'll use glycine, because it's simple and this is already hard enough as-is:

Now, we have the reaction scheme

$$nNH_3 + mX + \gamma \rightarrow NH_2CH_2COOH + jCH_4$$

We can (stoichiometrically) satisfy this reaction using propinoic acid

$$NH_3 + CH_3CH_2COOH \rightarrow NH_2CH_2COOH + CH_4$$

Based on a quick look at the standard enthalpies of formation, this reaction should be endothermic*. Check. Glycine is a relatively stable solid (we produce it all the time in our bodies) that is moderately complex (complex enough to be used to build proteins at least). Check. We're consuming ammonia and producing methane, as you asked. Check.

So this is my submission for your photosynthesis. The next step is to come up with whatever kind of crazy pathway this reaction schema could possibly use. However that sort of thing is way over my head (even photosynthesis on earth is really very complicated, involving lots of electron transfer and stuff), so this is where I'll leave you.

Happy worldbuilding!

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$*$ The enthalpies of formation I found are as follows (rounded quite a bit):

• Ammonia, -45 kJ/mol
• Propinoic acid, -510 kJ/mol
• Methane, -75 kJ/mol
• Glycine, 1430 kJ/mol

Thus the overall reaction has (45 + 510) < (1430 - 75) which implies it will not be spontaneous, with a net endotherm of about 800 kJ/mol. I believe this is a bit under half the endotherm of photosynthesis on Earth.

Answer 3

You only have one option: hydrogen. And that would work fine. Your photosynthesis is the reverse of photosynthesis in an oxidizing environment.

Hydrogen dissolves in ammonia. Hydrogen gas would be available in your reducing atmosphere, floating around in equilibrium with the ammonia.

Your heterotrophic creatures "eat" long chain alkanes by maximally reducing them to methane with the hydrogen - just as in our oxidizing environment we eat long chain carbohydrates by maximally oxidizing them to CO2 with oxygen.

Your autotrophic photosynthesizers produce their alkane food by stripping hydrogen from CH4 and forming long chain alkanes. CH4 + energy -> H2 + C_x H_2x+2.
Just as earth plants strip oxygen from CO2 to produce sugar.

Answer 4

Today I learned that photosynthesis must result in a complex compound. Well, I'm not a biochemistry expert, so learning is to be expected.

Therefore, there are 3 constraints on the photosynthesis reaction. 1. Results in a complex molecule. 2. Uses a minimum of oxygen, since the only accessible oxygen is in water ice, which is hard, relatively, to get 3. Uses Nitrogen to store energy.

I've come up with the following equation:

8NH3 + 3C2H4 + 2CO2 -> CONHNH2NOHN2 + 6CH4

As you can see, it uses a minimum of oxygen, results in a moderately complex compound, and uses plenty of Nitrogen.

Ethylene must be on the reactants side, otherwise there would be no place to dump the unusable excess hydrogen. French hydrogen would escape the atmosphere and therefore make this world too open of a system.

Additionally, like glucose, CO2N4H6 (the food product), can bind with itself to form chains. This is extremely useful as both an energy store and structural material.

Using average bond energies, this reaction needs 1352 kj/mol to proceed, comparable to Earth's photosynthesis which needs 2801 kj/mol to proceed.

But wait! That's not all! The compound here, whose name I do not know, has a better energy density than glucose. Per gram if glucose made, it yields 11.7 kj. However, this chemical, CO2N4H6, yields 12.7 kj per gram.

Note: The resulting compound is like Carbohydrazide except that the hydrogen on one of the Nitrogen bonded to the carbon is replace with a hydroxide group.

Hope this is a better answer.

Thanks to Ilmari Karonen for his comments on making this realistic.

Answer 5

Cyanide world.

Your other question Appearance of an Ammonia World's Atmosphere got me thinking more about this.

Here is my new idea for the cycle.

1. Photosynthesis: energy + N2 + CH4 -> HCN + NH3. Energy is stored in the CN triple bond.

2. Heterotrophs: NH3 + CN -> CH4 + N2

I admit I am struggling mightily with the enthalpy of the heterotroph part. I would post to the chemistry stack but I am sure they will feel it is below them. Any help with that most welcome.

Then the cellulose / sugar equivalent is nitrile rubber!

https://en.wikipedia.org/wiki/Nitrile_rubber

A good polymer for a low oxygen world, and a good sugar equivalent.