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A planet approximately 70% the size of Earth with a surface pressure of approxiamtely 0.25 bar at sea level. The atmosphere consists of the following gases:

  • Nitrogen (98%)
  • Oxygen (1.5%)
  • Carbon monooxide and dioxide (0.5%).

The planet orbits a red dwarf at the outer border of the habitable zone, with average surface tempatures ranging from -50°C to 0°C.

My questions are:

  • How would the sky on this planet appear to an observer located at sea level during a.) midday and b.) sunrise/sunset or dusk?

  • How would the atmosphere of the planet appear when observed from orbit?

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    $\begingroup$ Just a note: oxygen is a highly reactive gas and carbon monoxide is no slouch itself. Those gases cannot simply "exist" in the atmosphere -- there must be some process which replenishes constantly them or else they will find something to react with and be gone. Not to mention that carbon monoxide is combustible: oxygen will combine with the carbon monoxide giving carbon dioxide. $\endgroup$ – AlexP Mar 3 '17 at 15:41
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Since your planet orbits a red dwarf, the light hitting it from the sun will be mostly red. And since its atmosphere is quite thin, the light won't be scattered as much and the sky will appear quite dark even during the day. Any particles suspended in your planet's atmosphere will also affect the way the light is scattered, and thus, the sky's colour.

I can't claim to be 100% certain what this planet's sky will look like, but based on the first two points, I'm thinking it would most likely be a dark red or maroon colour during the day, slowly fading to black as it sets. In other words, pretty darn creepy.

(sources: this question and this question)

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    $\begingroup$ Red dwarf light is not as red as you think it is. The surface temperature of red dwarf stars is between 2,300 and 3,800K. An incandescent light bulb filament is at a temperature of around 2,400K, so most red dwarfs are less red than a light bulb. $\endgroup$ – Mike Scott Mar 3 '17 at 12:59
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    $\begingroup$ Earths sky is blue because of en.wikipedia.org/wiki/Rayleigh_scattering. Cyan/blue light scatters better in Earths atmosphere and in general. Complete removal of blue leaves yellow on the additive color wheel, which is why the sun is mostly yellow in the sky. As with what Mike said, Almost all stars will appear white to the human eye due to how bright they are, and the fact that they do emit just about every color in vast amounts. $\endgroup$ – Ryan Mar 3 '17 at 17:25
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An atmosphere tends to scatter shorter waves, so the default sky colour is violet. (It's blue on Earth because the Sun spectre is short of violet).
Red stars can have complex spectra with violet and blue waves, so your sky might be violet.

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  • $\begingroup$ ...and because we cannot see violet as anything but blue :D $\endgroup$ – MichaelK Mar 3 '17 at 13:19
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The Earth's atmosphere also consists mainly of nitrogen, although only 80% rather than 98%. However, I think oxygen and nitrogen have similar scattering properties, so the exact mix shouldn't matter much.

The sea-level pressure for your planet is about the same as airline cruising altitude on Earth. So if you crane your neck and look up you can see the deep sky blue color that people would see if they were orbiting the Sun.

The fact that the star is cooler means, I think, the the sky will be a bit darker, but not redder. The "redness" of the star really means that it is less blue than the Sun, and since it's light at the blue end of the spectrum that the air scatters, you will just get less of that.

But even "darker" might not be how it's perceived. The paleness of sky blue comes from the fact that there is a mix of light from other parts of the spectrum in there. If you dial that down while maintaining the same proportions, the eye will adjust and simply see a dimmer pale blue sky.

In general this is hard to get right. Before the Viking landers, a lot of popular science literature predicted the Martian sky to be dark, perhaps navy blue, because the atmosphere was so thin. But it's pink, or at least a butterscotch color. However, I think you should assume a variant of sky blue until proven otherwise.

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Take all I say with a grain of salt.

First, you may want to look at the sky chart I made many months ago as it directly answers your question: Terrestrial Exoplanet Skies – I've Built a Visual Sky Chart. Is it Correct?

Referencing that chart, it will probably look more similar to earth's sky than you would supposed. You're describing an atmospheric composition that is very earth-like. The biggest difference would probably be the low-atmospheric pressure. It would be like standing on a mountain-top on Earth (but more so) where the sky colors are noticeably darker due to less scattering of light. So all your colors will be darker (the sky will be less "bright" in color). The general principle is: Lower pressure darker colors, higher pressure more muted, "whited out" colors.

The other big difference is the red dwarf. You haven't specified what class of red dwarf (and this really matters) so I'll say that your sky would almost certainly be less blue (more white/grey component) than Earth's sky. This is due to less long-wavelength light reaching the surface. Though contrary to popular perception red dwarf light is still whitish light, it's just has less blue and more red wavelengths than Earth sunlight. If it's a really dim red dwarf (like an M8) your sky might take on a somewhat orange/brown tinge even at noon (due to sever lack of longer-wavelength light).

Noon: A darker, greyish-blue (maybe brownish-orange) at zenith getting whiter/grey towards the horizon. The sky overall is much darker in color than Earth's.

Sunset/Sunrise: Various colors depending on clouds and other factors (just like on Earth), but in general the spectrum will shift more red towards the setting/rising sun, and the sky on the opposite horizon will likely stay blue-ish but much much darker. Overall the main difference will probably be it's just a darker sky overall (thanks to the low atmospheric pressure).

If you have dust in the atmosphere, certain trace gasses, or other things than that could greatly effect appearance.

As for what it would look like from space... pretty much how Earth looks I suppose. I don't see a reason it would look much different, other than the landforms and oceans taking on a slightly more red-tinge. And I do mean slightly. There is less blue light making it to the surface, but blue light is not absent. It's the difference between a 3000k bulb and a 5000k bulb...noticeable, but not so much that your blues, greens, etc. completely vanish.

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Here is Nitrogen spectrum http://s3.amazonaws.com/everystockphoto/fspid31/34/90/75/6/aurora-nitrogen-emission-3490756-o.jpg
As you can see there is a lot of red. And because red dwarf is in the infrared emission you would have red everything. It would look like thermal image but without yellow. I can't tell if the "light" areas would be black (no light bounce back) or close to the orange.

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  • $\begingroup$ I believe the picture shows a spectrum of aurora, northern light, so it can only apply to highly ionized atmosphere or something $\endgroup$ – user8808 Mar 3 '17 at 15:38

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