View Full Version : Under alien skies

2006-Jan-12, 10:20 AM
In some pulp sci/fi novels, the author tries to enhance the alien setting by changing the colour of the sky to something different than we see here on Earth. Nothing speaks of an alien planet like pink clouds under a green sky... However, is this possible?

In other words, is there a combination of stellar spectrum or atmospheric composition that would result in e.g. a green or purple sky and still remain breathable? I don't know enough about how Rayleigh scattering works to be able to tell.

2006-Jan-12, 03:23 PM
Planets with breathable atmospheres will probably be very much in the minority.
If a very-Earth-like planet were to orbit a much redder star- say a red dwarf- then the colour of the sky might be quite noticeably different. I would like to think that the sky would be lilac or mauve on such a planet; it appears more likely that the sky colour will tend more towards the white or yellowish, as there is less blue light to scatter.

However some non- earth-like planets with life but with different atmospheric densities and might end up with skies that look quite different; suspended iron rich solids could give a reddish tint like that found in Mars's sky, or suspended phytoplankton could give a green cast to the sky. If an atmosphere is sufficiently dense phytoplankton could remain suspended in the air just as algae are suspended in Earth's Oceans.
Also organic molecules like Tholins could add other, brown or orange colours.

Mars has a butterscotch coloured sky because the atmosphere is so thin that blue Rayleigh scattering is not noticable; on a planet with Earth-like atmospheric density the blue would be there too, leading to a blend of butterscotch and skyblue colours;
a dusty blue sky- pale colours like in a desert on Earth (probably).
Coloured gases like chlorine might add to the palette- but you couldn't breathe such an atmosphere...

So quite a few different colours available, but only blue from Rayleigh scattering really.