About the color of atmospheres.

Hi, I want to start this interesting discussion or speculation of atmosphere colors related to the proportion of gases. I'd like to start with an earthlike atmosphere.

We all know that earth is blue due to the amount and proportion of gases. The main gas is nitrogen and the second is oxygen. A pale blue dot. But what if we exchanged those gases, I mean, what if an earthlike planet had the same amount of oxygen that Earth was with nitrogen? Does the sky would still look blue or would be a little different? And what if Earth had the same atmospheric pressure but the main gas was carbon dioxide instead of nitrogen? In any case, what would be the color of sky if we had...

1 atm pressure: 90% N2, 7% O2, 2% CO2, 1% others like H2O and CH4? (earth-like oxygen poor planet)

1 atm pressure: 90% O2, 7% N2, 2% CO2, 1% others like H2O and CH4? (earth-like oxygen rich planet)

1 atm pressure: 75% N2, 21% O2, 3% Cl2, 1% others like H2O and HCl? (earth-like planet with clouds and oceans of water/hydrochloric acid)

1 atm pressure: 85% N2, 10% NO2, 3% CO2, 1,5% NH3, 0,5% H2O? (earth-like planet with clouds and oceans of ammonia/water)

1 atm pressure: 50% CO2, 25% O2, 23% N2, 1% Ar, 1% others like H2O and CH4? (earth-like carbon dioxide rich planet)

1 atm pressure: 80% CO2, 10% N2, 7% O2, 2% H2S, 1% SO2 and H2O? (earth-like similar to Venus but with sulfuric acid clouds and ocean)

1 atm pressure: 90% H2, 9% He, 1% water? (earth-like hydrogen rich planet)

Of course, assuming that in all cases it's a planet orbiting a G2V star like our sun.

For now, these are my questions about this subject.

The reason for those questions is because I'm developing a Scifi book about a fictional planet system in Andromeda around a 100 sun mass black hole and a G2V star...

I don't know how to answer your questions, but this might be an interesting starting point.
http://panoptesv.com/SciFi/ColorsOfAlienWorlds/AlienSkies.php

This article explains in detail the color of the sky. It uses the Nishita sky model which reproduces the sky color with the most accuracy.

In summary, the color of the sky is governed by two atmospheric scattering models: the Rayleigh scattering and the Mie scattering models.
The Rayleigh scattering is used for molecules that are much smaller than the wavelength of the incident light (ie: sunlight), it's what gives the sky on Earth its cyan color. The Mie scattering is used for molecules with sizes equal or greater than the wavelength of the incident light (ie: aerosols).

I don't know much about Mie scattering but for Rayleigh scattering it works like this. What really matters is the size of the molecules, the density of the atmosphere (at sea level), the temperature of the star and the time of day. Here I am assuming that the time of day is around noon and a sunlike star.

The earth sky is cyan in color because the atmospheric pressure is 1 atm and nitrogen and oxygen are molecules smaller than the wavelength of the sunlight. Increasing the atmospheric pressure will make the sky appear blue, and purple with a very high atmospheric pressure. Lowering the atmospheric pressure will make the sky appear white, then yellow, then orange and finally red. It is one of the reasons why the sky on Mars is orange/red (the atmospheric pressure is very low).

The colors for your atmospheres would be as follows:

1 atm pressure: 90% N2, 7% O2, 2% CO2, 1% others like H2O and CH4? (earth-like oxygen poor planet) -> cyan

1 atm pressure: 90% O2, 7% N2, 2% CO2, 1% others like H2O and CH4? (earth-like oxygen rich planet) -> cyan

1 atm pressure: 75% N2, 21% O2, 3% Cl2, 1% others like H2O and HCl? (earth-like planet with clouds and oceans of water/hydrochloric acid) -> cyan with white/yellow clouds maybe

1 atm pressure: 85% N2, 10% NO2, 3% CO2, 1,5% NH3, 0,5% H2O? (earth-like planet with clouds and oceans of ammonia/water) -> cyan with white clouds

1 atm pressure: 50% CO2, 25% O2, 23% N2, 1% Ar, 1% others like H2O and CH4? (earth-like carbon dioxide rich planet) -> cyan

1 atm pressure: 80% CO2, 10% N2, 7% O2, 2% H2S, 1% SO2 and H2O? (earth-like similar to Venus but with sulfuric acid clouds and ocean) -> cyan with yellow clouds

1 atm pressure: 90% H2, 9% He, 1% water? (earth-like hydrogen rich planet) -> cyan

Interesting... Thought the sky of an ammonia planet with NO2 gas would appear orange or brown, due to the color of nitrogen dioxide, or maybe could be that way if the pressure was higher, who knows? And also thought the sky of a chlorine rich planet would appear greener, because chlorine gas is green. But maybe with a low amount of Cl2 in air, the sky would still look blue, but with some green near the horizon. In the case of a CO2 rich atmosphere, thought it would be yellow because Venus air is yellow and is very rich in CO2. But maybe that has more to do with the high pressure, and the presence of SO2 too. So, whatever the gases make the atmosphere, in most cases the sky will look cyan with 1 in pressure, or am I wrong? Later I'll bring more questions. But for now this is it.

In the case of Venus, the Mie scattering has the upper hand because the planet is fully covered with clouds (clouds are essentially made of droplets that are millimeters in size). Also, I don't think green skies are possible, simply because there are no stars appearing green.

Edit:
Maybe a green sky could be obtained by having a G and a K star orbiting each other. The G star alone would give a cyan sky, and the K star alone would give a yellow sky. Mixing cyan and yellow would make a green sky.

This time the question is not about color, but maybe have some relevance... This time the question is about earthlike moons around giant planets. I have some doubts like...

Will the atmosphere of such world, in the habitable zone around a sunlike star will have an atmosphere similar to Earth, or it's more probable to such atmosphere have more CO2 than N2 or O2? I'm making this question because, as far as I know, many earthlike moons will tend to orbit somewhat close to the parent planet. In this case, depending on the mass of the planet and the distance to it, it will tend to be a volcanic moon. I imagine most earthlike moons bigger versions of Io with atmosphere, oceans and life. It's like mixing Earth with Io. If the moon is well protected from stellar wind, maybe it will tend to have a dense atmosphere, even with marslike mass, with lot's of CO2, like mixing Mars with Venus and Earth in this case. So assuming this, I think most earthlike moons will tend to have a lot CO2 with N2 and O2 in lesser amounts, due to continuous volcanism caused by tidal forces. I imagine that most earthlike moons will be more like Io-Mars with oceans, continents and plate tectonics and the most distant earthlike moons will be geologically quieter worlds, with less volcanism, so less CO2 and more N2 and O2 in the air.

I will create here a scenery. Imagine a giant planet, called Aquaron. It orbits a sunlike star, F, G or K type. This gas planet have 3 jupiter mass and is blue with white because it has hydrogen air with water clouds. It has 2 earthlike moons and several other smaller moons, with a saturnlike ring system made of ice, rock and dust. The rings have bluish color because of the reflected blue light of the planet on those ring ices and rock. A very beautiful sight indeed! The ideal gas giant type for habitable earthlike moons in habitable zone.

The first is called Nimbus and the second Teran. The first, Nimbus, have mass similar to Mars but is denser so has more gravity and escape velocity, so it can hold more air. Due to the smaller distance to Aquaron, Nimbus have strong volcanic activity, but a lot of it's surface is covered in water (90%). But those oceans are not so deep and so can't hold much CO2, that escapes to the atmosphere, unlike Earth. The air pressure is about 3x earth's the air composition is: 50% CO2, 30% N2, 17% O2, 1% H2O, 1% H2S, 0,5% SO2 and 0,5% Ar. Average temperature is 50°C due to all that CO2. The higher amount of H2S and SO2 are due to the intense volcanism. But local life evolved to use and tolerate those gases along with CO2. Due to the high humidity and water content, the sky is mostly covered in clouds, which help cool the air, so the name Nimbus. If was not for all that cloud cover, temperatures could be as high as 75ºC or more. The higher air pressure prevented a runaray greenhouse effect as happened on Venus. Local life is mostly aquatic and there is an intelligent anfibian that created cities in coastal areas in both land and underwater.

The second moon, Teran, orbits at a further distance. It has earthlike mass and because it's further, despite being more massive, volcanism is far less active, but still as powerful as on Earth. Most of it's surface is water like here too, but also has much land (60%). The oceans are deeper unlike in Nimbus, so they can hold more CO2. Because of it the air has far less CO2, but still in higher amount than on Earth, because of the moon's own volcanism plus the tidal effect. Air pressure is 6x Earth's and the composition is: 84% N2, 10% O2, 4,5% CO2, 0,5% H2O, 0,5% H2S 0,5 other gases. Average temperature is 25°C. Despite the higher amount of gases, it has far less CO2 and greenhouse gases, so in this case temperatures are lower and also because of certain cloud cover, which is still higher than on Earth but not like Nimbus or even Venus. The lesser amount of cloud compared to Nimbus lower the albedo, increasing temperature. If it was not for all that high pressure, temperature would be so cold that Teran would be like Earth in ice age, at around -25°C. Local life is terrestrial very similar to humans and live in a way very similar to ours.

Due to the further distance to Aquaron and also due to the interaction with other moons, it rotates like Earth but at a very lower speed. In this case the day lasts around 1/10 the orbital period around the planet, while Nimbus is tidal locked due to the closer distance. Teran has it's own magnetic field driven by it's own rotation and also by the tidal effect from Aquaron (that here is small). Nimbus also has this too. The day in Teran lasts around 72 hours and the orbital period lasts 30 days. So for one translation around the planet, it takes 10 teranian days. Add the period around the star and that would make a very interesting calendar! Despite the higher distance to Aquaron, Teran won't escape the planet hill sphere due to the high mass of the planet. The hill sphere is comparable to that of Jupiter, maybe bigger. Maybe the moon might go closer to the planet in time, but certainly it will end up tidal locked with a more stable orbit, because Teran is in fact a captured planet and did not form with Aquaron. It's destiny is uncertain, but certainly it won't get too close to the planet, because of Nimbus interference.

There's another moon called Ozinera. This moon is between Nimbus and Teran and is small like our moon. But it's special because it's very similar to Europa, but with mountains breaking through the ice layer to very high altitudes, like mountains on Mars or Io. It's atmosphere is very thin made of mostly oxygen and ozone with traces of other gases. It's pressure is very lower than on Mars and the temperature is -100°C on average due to the high albedo. Under the very thin ice layer there are shallow oceans rich in volcanic activity and also smokers. Aquatic life is rich, but less than on Nimbus. Photossynthetic organisms live right under the ice layer, so in Ozinera there are huge algae forests right under the ice. Those algae are feeded by the light that penetrate the thin ice and open cracks and also feeded by minerals coming from the thermal vents right under them, because the oceans are shallow here. But this world might die with time, because the ice, due to radiolysis is being broken and with that the oceans are becoming even more shallower, to the point that there will be no more them. It's a dying world like Mars in the past. But this will take billions of years to complete. Until there, intelligent life might evolve and make it's way to survive that cathastrophe.

I imagine this scenery for most habitable earthlike moons in the habitable zone. I imagine Nimbus to have a very pale blue atmosphere due to the high content of water and all those clouds. Maybe the air color on the surface could even be white, like an overcast day on Earth, but constant all year around. In the case of Teran, the sky might be more similar to Earth's but also more pale due to the higher pressure. And in the case of Ozinera, the sky will be a very deep blue, because it's very dry. Or if it's rich in ice crystals will be a bit more pale blue, but still very dark, more than on Mars because it almost does not have dust in the air.

And finally... The bigger reason for this is because i'm writing a scifi book, with many planets and moons around a huge black hole (100 solar masses) with a very hot and bright accretion disk and 2 stars, a G2V and a M2V star. Both being slowly sucked but in a very stable orbit, in a way that the accretion disk will continue to shine and heat planets and moons for many billions of years ahead. And the habitable earthlike moons at a very safe distance in many AU away, so that those moons won't toast or lose their atmospheres. In fact those 2 stars will tend to lose mass so they will never go red giant like our sun. The tendency is that with time the entire system will cool with time, and not get hotter and engulfed by a star as will happen to Earth. Life on many worlds will have plenty of time to develop, in fact much more than 10 billions of years. This is a system of a very energetic light source that will last much more than our sun, better than most of those dark and energetic inefficient red dwarves and better than those short lived blue stars.

A light source that will last long as a red dwarf, but that is energetic as a blue dwarf star. Just the perfect system archtecture!

The very perfect scenario for life to survive for very very long time and have the luxury of an energetic light source. Yeah, black hole bright suns!