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I started the process of making artwork for an Ammonia World and realized that despite all my previous research on their abiotic and biochemistry, there was one thing I never quite settled on; what the atmosphere should actually look like.

Air Pressure: ~2.8 Atm

Composition: 65% N2, 24% NH3, 10% CH4, 1% C2H4, NH3 partial pressure highly variant on temperature

Temperature: About -60 to -50 C

Gravity: ~1.2 Gees

Parent Star: K Type

On the one hand, there's a lot of methane, which tends Uranus and Neptune blue. On the other hand, aside from the ammonia, this is similar to the chemical makeup of Titan, which has that deep tholin haze that makes everything orange. This also differs from Titan in that the air pressure is higher, the sunlight is stronger, and the gravity is higher, so the actual air column height should be much more compact.

I figure there should still be some photochemistry going on in the upper atmosphere, but I'm not sure if this would make the planet as hazy as Titan. There are other questions on the stack that discuss methane-rich ammonia worlds, but I haven't seen the issue of photochemical haze come up. My intuition is that the denser air and more compact column might produce a thinner haze layer in the upper atmosphere, not thick enough to turn the atmosphere opaque, but I have no idea if this is right or if it would affect the sky color. This could be good though if right, as I've seen papers suggesting tholin hazes on Early Earth may have screened UV light, which would be a boon for a life-bearing planet with no ozone.

So, thoughts? Blue sky? Uranus-like light blue? Orange? Blue with really orange sunsets? I've spent the entire evening wracking my brain about this and can't figure this out, and it bugs me.

I got the below in Terragen by assuming a bit of haze and giving the atmosphere an orange "Haze Horizon Color". Looks interesting, just not sure if right.

(not relevant to question, for for the curious, those are black grass-like plants on the right) enter image description here

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Black clouds of cyanide polymers.

In the nitrogenous, oxygen-poor atmosphere of your world, photobiology would favor formation of cyanide.

Hydrogen cyanide in nitrogen-rich atmospheres of rocky exoplanets

photochemistry depending on oxygen

Cyanide is unstable in the presence of oxygen, which can compete off the nitrogen to form CO. Your world is oxygen and water poor and so cyanide would form and persist.

Cyanide is soluble in ammonia and dissolved cyanide can polymerize, forming red, brown, yellow and black solids.

Dark matter in the solar system: Hydrogen cyanide polymers

In the presence of a base such as ammonia liquid HCN (bp 25 °C) polymerizes readily to a black solid from which a yellow-brown powder can be extracted by water and further hydrolyzed to yieldα-amino acids. These macromolecules could be major components of the dark matter observed on many bodies in the outer solar system. The non-volatile black crust of comet Halley, for example, may consist largely of such polymers...

Maybe in your world it would just be tholins as you say. But maybe with no water the chemistry is impoverished - cyanides and nitriles. Polymeric cyanide soot clouds would blow through the air of your planet, crackling with lightning. Black drifts of the stuff would blow across the surface.

In some places on the surface, long CN polymers wetted by liquid ammonia or alkanes would be gooey and thick, and accumulate in deep pockets. It would be hard to figure out where these places were because they would be covered by the same soot drifts. But you would want to figure out because you would not want to fall in. Such pockets would be prime places for life to form.

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  • $\begingroup$ I would love to see how this looks in Terragen. $\endgroup$
    – Willk
    Jun 1 at 1:42
  • $\begingroup$ Somehow I got too distracted to actually check the important bit. Assuming an upper atmosphere haze of these cyanide polymers formed, would that tend to make the sky more orange, even if it was an overall thinner one? $\endgroup$ Jun 9 at 21:41
  • $\begingroup$ @SamD.Jones - probably. The most likely outcome is orange tholin sky. But I thought the billowing soot clouds was an interesting change. $\endgroup$
    – Willk
    Jun 9 at 23:06
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Impurities will carry the day.

Like rubies and sapphires, your atmosphere is made mostly of colorless materials. If nothing else is present, the sky of Earth and exoplanet alike can have a blue tint due to Rayleigh scattering (scattering from things much smaller than a wavelength of light). Mars, on the other hand, has a red sky with a bluish tinge near the Sun due to traces of iron (III) oxide. On Titan you mention the tholins (or cyanide polymers, as nicely detailed above).

The impurities are up to you, endless possibilities, but what appeals to me at first glance is the notion of "solvated electrons". If you dissolve sodium in ammonia, the solution turns deep blue as the Na+ and electron separate - the electron wanders among a cluster of ammonia atoms as if it were almost free. Weirdly, this is thought to happen even in Earth's atmosphere! This involves small clusters of water molecules. It looks like something similar may happen in ammonia gas phase.

Now I'll admit I'm past the end of my tether when it comes to predicting what could happen here, but you have a very large amount of ammonia, a highly reducing planet, and a range of temperatures that allows for saturation of the air and precipitation. Should you go in a direction where reactive alkali metals might be found in reduced form and be able to weather as single atoms, maybe it's plausible that this could lead to strange deep blue coloring of the winds passing over these regions?

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