I ran across an interesting Orion’s Arm page, which provides a page on the sky colours of terrestrial worlds on alien stars.
Copying over the pretty parts, and leaving the boring and difficult science and graphs alone [and my comments in square brackets]:
(Above graphic created by Steve Bowers)
Four different sky colours caused by different amounts of Rayleigh and Mie scattering in increasingly dense atmospheres, for planets orbiting a Sun-like (G-type) star. From the left:
a) A Mars-like world with a thin atmosphere of less than 0.01 bar; the amount of Rayleigh scattering is minimal, but particulates suspended in the atmosphere colour the sky by Mie scattering and selective absorption.
b) An Earth-like world with an atmosphere of one bar; the colour ‘sky blue’ predominates because of Rayleigh scattering, getting paler towards the horizon.
c) A world with a pure, particulate free molecular atmosphere of 10 bars; the sky colour is cyan, with the horizon considerably paler.
d) A world with a pure, particulate free molecular atmosphere of 40 bars; the sky colour is pale yellow, with the horizon almost white.
Unexpected Sky Colours
Surprisingly, such a simple detail as the colour of the sky on an alien world is far less than obvious. There are three reasons for this: incomplete knowledge concerning the workings of the human eye and the perception of colour, a very broad range of possible alien atmospheres, and of course simple lack of experience. Though the fundamental physical principles are all well known, their application is less than obvious. Mars’ pink skies were a surprise when they were first seen.
Eyes and Appearances
The appearance of the sky depends very much on the equipment used. For instance, humans see radiation in the 380 to 760 nm range (bluish purple, to blue, to green, to yellow, to orange and red, and to reddish purple). However, even within this range we do not perceive colours equally. We actually have three sorts of receptors in the eye: one with a peak at red, one with a peak at blue, and a third with a peak at green. Other colours are known by interpolation. For instance, a computer screen never gives off yellow light, and it doesn’t need to for a human, since the human brain assumes yellow from the balance of green and red light it perceives. Similarly, both edges of our range look “purple”, which is what we read when light reaches us but isn’t strongly affecting any of our colour receptors. What we call brown is actually a mental construct, based on a stimulus to our three kinds of colour receptors.
Other vertebrates see colours over approximately the same range that humans do. Wavelengths shorter than 300 nm and longer than 800 nm cannot be detected with a photochemical eye based on organics, since shorter wavelengths destroy the material and longer wavelengths are not powerful enough to affect it. However this does not mean that other animals see the colours that humans do. Most mammals have only one or two sorts of colour receptors in the eye and presumably distinguish fewer colours. Many birds and fishes may have greater colour definition since they have four or more types of colour receptors. The bird’s extra receptor is in the ultraviolet range, adding an entire range of colours of which humans are unaware. Many insects have three receptors, like humans, but one of the three picks up ultraviolet. What these organisms see when they look at the sky is a matter of speculation.
[Well, I did say “the boring and difficult science”. The interesting and cool science is welcome anytime!]
Earth’s own turquoise blue sky, as seen by humans, comes primarily from the Rayleigh effect.
[Only a briefest of mentions in this post: the curious can go to the original page, or hit Google for more..]
The Size of the Atmosphere
Another effect is the total amount of atmosphere. If there is no air at all then the sky will be black, as it is on the moon, since no light is scattered. It acquires more and more of whichever colours it would otherwise have as it thickens. This can be seen at high altitudes on Earth; the sky becomes a darker and darker blue until it becomes entirely black. What happens as the atmosphere thickens is something one can see at sunrise and sunset. If the sunlight goes through enough air then the blue and green light are so scattered that you start to see the reds and yellows, and the sky has a golden or yellow hue, pink, and then red.
[If, somewhere in the 2040s, someone makes a first-person world generator for any (habitable) Traveller world, this should be kept in mind.]
Suspended Dust or Droplets
Dust or droplets in the atmosphere have a very different effect. If they are more or less transparent, like water droplets, or if they have no particular colour themselves, then they simply scatter all wavelengths of light equally. This is called Mies scattering, and it is the reason that a sky full of fine water droplets is a paler blue than a dry sky. At the extreme, of course, the sky is white or grey (i.e. cloud covered). If the dust or liquid is coloured, the sky will take on that colour.
[And that’s how you get a proper purple sky… and purple rain, too!]
Stellar Type and Kinds of Sunlight
(Above graphic created by Steve Bowers)
Sky colour at zenith on an Earth-like world under various different types of star.
Changing the illumination changes the sky colour yet again. Though a type G star like the sun has a peak in the green/blue range, a type M star has a peak in red, K peaks in orange, F stars are brightest in the blue, and A and higher are brightest in the ultraviolet range. For this reason the stellar class affects the colours of planetary skies significantly.
[Me? I’m just glad that a M-class red star doesn’t mean a scary red sky. Of course, a bit of though would demonstrate that our yellow star does not mean a yellow sky…]
An Earth-like planet circling an F-type star will have skies of a vivid blue, with scattered blue light making up 77% of the total perceived colour; planets orbiting G-type stars have ‘powder blue’ skies, with blue light making up 70% of the total. A typical K-type star will give a light blue colour with a 61% blue component to the sky of an orbiting Earth-like planet, while the most common type of M-type red dwarf will impart a pale-whitish blue colour to the skies of such a world, with no more than 44% blue in the mix.
Of course the effects of atmospheric density and the effects of spectral class must be taken together. For instance a planet with a colourless, dense atmosphere orbiting a red dwarf will have a sky which is almost completely white; clouds on such a world would reflect the unscattered light from the star and appear yellowish or reddish. Any coloured gases or particulates will affect the colour still further.
On a world that has life forms, the sky colour might be affected if enough of those life forms are airborne. If for instance the photosynthesizing organisms used chlorophyll this is one of the relatively rare circumstances that would result in a green sky.
Sky plankton may occur on several different classes of world, including gas giants, terrestrials and superterrestrials.
[Not so great for air-breathing engines without proper filters. And would such plankton ruin the PCs fuel purification system? Probably not, but there’s only one way to find out!]