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The main character in my novel-in-progress is an alien whose home planet has a yellow sky. This planet orbits Altair (an A7 main-sequence star) and has a nitrogen-oxygen atmosphere similar to Earth's, with a few notable differences:

  • A much higher level of water vapor, resulting in the planet being mostly cloud-covered. (The clouds are not yellow, but clear sky, when it shows, is.)
  • Higher barometric pressure. I don't have a numeric figure, but it isn't extreme. An inhabitant of this planet would likely suffer altitude sickness at the same elevation on Earth but would be unlikely to develop serious complications.

The air doesn't have to be human-breathable, but I need the reverse to be true. Toxic gas(es) would be pretty cool, but I don't know if it's plausible for the native oxygen-breathers to be unaffected. I've been told any particulate matter would settle when it rains, which isn't what I want; I'm looking for a permanently yellow sky. I've also considered the possibility of airborne microorganisms, possibly above the cloud layer (and thus not disrupted by rainfall), but I have no idea if that's plausible.

Is a yellow sky possible, given these constraints? How?

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Yellow aurora.

yellow aurora

https://www.lemurialight.com/blogs/amazing-life/yellow-aurora

Yellow is one of the colors possible for the aurora on Earth. Yellow is caused by charged paticles in the solar wind striking oxygen high in the atmosphere. The oxygen then emits yellow light.

This happens constantly on your planet. Because of the near constant haze, the pure yellow light emitted by oxygen is diffracted in the haze, such that the entire sky glows yellow.

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    $\begingroup$ This is a wonderful idea! (Actually I don't think it would work in reality, but you could easily handwave it with a peculiar combination of the star's remarkably steady solar wind and atmospheric composition...) $\endgroup$
    – LSerni
    Sep 25 '20 at 10:59
  • $\begingroup$ I love this idea! Thank you! $\endgroup$ Sep 27 '20 at 23:28
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The air colour is caused by a physical phenomenon, the Rayleigh scattering. Absent this, the air away from the Sun would appear black, since there would be no light coming from directions others than the Sun's (basically the same that happens in space, or going high enough in the stratosphere: from the directions where no direct light source exists, we see no light, and therefore -- black. With stars).

Increasing the pressure will cause more scattering, the Sun will appear redder and the sky a stronger blue. Also, more light will be absorbed, so a darker shade of blue too (and finally black again, but without stars this time. You'd need a lot of atmosphere to do this, though; or lots and lots of suspended particulate matter, as in a nuclear winter).

The light arriving to the "far sky" has no appreciable yellow component. So, we need something to actively produce yellow light, for the sky to appear yellow. Willk's answer has a very elegant solution to that.

The only way out I could see was your idea of a dispersed chemical. This would absorb the ultraviolet band, of which an A7 star has lots, re-radiating in the 580nm yellow band either through frequency halving or, much more common, yellow phosphorescence. Possibly some simple sulfur compound might do, and this could be continuously supplied by volcanism (the sky did have a yellowish tint after the Krakatau eruption, it is said).

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    $\begingroup$ What you mean by "the air away from the sun would appear black" is that, without the effect of Rayleigh scattering, the air would be entirely transparent, allowing us to see the black of space when not looking at the sun. Right? $\endgroup$ Sep 23 '20 at 22:31
  • $\begingroup$ @JBH yes, that's it. Amending answer $\endgroup$
    – LSerni
    Sep 24 '20 at 7:16
  • $\begingroup$ I am a little confused about the absence of yellow. The sun appears yellow-ish in the sky due to the blue being scattered out of its mostly-white light, yes? And there are yellow sections in sunsets? Why wouldn't making the atmosphere as thick as it is in the yellow section of sunsets, but at mid morning/afternoon angles instead of sunrise/sunset angles, do the trick? $\endgroup$
    – CAE Jones
    Sep 25 '20 at 7:45
  • $\begingroup$ @CAEJones the sun's yellowness comes from subtraction. You take away the blue light, that gets scattered all over the sky, and you're left with yellow light from the Sun. If you increase the quantity of atmosphere you get just more of the same - a redder Sun and a darker-blue sky. You are absolutely right in that an area around the Sun will scatter appreciably yellow and red, but that's not all the sky. $\endgroup$
    – LSerni
    Sep 25 '20 at 10:56
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So the color of the Sky has a lot to do with Light Scattering. One Major factor for the scattering is the thiccnes (with double c !) of the Medium.

enter image description here

As you can see here. Now what determainds the Color of the Sky on Sea Level is the "Base Color". For example, if you have a Base Color of a Light Blue, you get this:

enter image description here

You will always end up with a Gradient no matter the base color. And lastly, another major factor is the Density of the Air. Lets incress the density to 5 Units in this example:

enter image description here

So the main give away is that Density and Hight can be used to change the Gradients Intensity. You can get the color you want with both, but as you see, a higher density compresses the Gradient. And is also more Realistic. The renders here dont take into account that the Atmosphere has a Falloff though. So Lets add that.Same base color but an Exponential Fallof for Density:

enter image description here

You can see how this affects things. To make a long story shot, for your Sky to be Yellow, you want the most right part of the Renders to be yellow. For that, we need to change the base color. This could work:

enter image description here

To no one's supprise, an Orange Yellow with this HEX FFD115 is a good choice. But now to the interessting part, what gases would we need ? Well...

That is not so easy to say. See, Or Sky is Blue because the Blue Wave lenghts dont get Scattered that much. That is why the sky is Red during dawn because more Blue light gets scattered. So the Color actually mostly depends on the Density of the Atmosphere, and not so much the Gases inside of it. Sure they do play a big role but in the end, it matters how long the light has to travel. Or through how much. Venus and Titan both have Yellow skys. But if you Look at the Atmosphere of Venus, you see that most of it is Carbon Dioxide with some Nitrogen. Both of which are Colorless.

To answer your question, is a Yellow sky possible ? Yes. For the most part, the Density of the Atmosphere creates the Color so a very Dense atmosphere will create a Yellow sky on the Sea Level. But the Amount of Pressure needed is sort of High. You could go a different rout and just make the Atmosphere very Larg, but this creates even more problems.

But, since you create an Alien world, what is there to say that they didnt just evolve for such high Densitys?

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    $\begingroup$ I'm afraid this is incorrect. Titan's sky color is orange, not yellow, and it's that color not because of scattering, but because there's so little light the effect of scattering is overwhelmed by the dust in the atmosphere (which is why the skies of Oregon during the wildfires were orange). Rayleigh scattering always scatters blue because chemically it's easier to reflect than the other spectra - and it doesn't matter what gas we're talking about. Getting a yellow sky like our blue sky (through rayleigh scattering) isn't possible that we know of. $\endgroup$ Sep 23 '20 at 22:29
  • $\begingroup$ "Or Sky is Blue because the Blue Wave lenghts dont get Scattered that much." its the opposite isn't it blue light is scattered very strongly hence it seems to come from everywhere, while longer wavelngths are more direct so only seem to come from the sun $\endgroup$
    – jk.
    Sep 24 '20 at 7:40
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Your clouds are sulphur dioxide, your air is rich in S$_8$, odourless and tasteless, and chlorine (Cl$_2$) may make a contribution too; windborn iron-rich dust intensifies the yellow tint.

Your star is probably K-Type.

Source: https://www.youtube.com/watch?v=L9MNC45Jr6Q

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