Sky colour

[Warren_Paul]

Since I've started writing a sci-fi, I'm wondering about the colours of skies on other planets. Personally I'd like my habitable planet to have a colour other than blue, like orange maybe. But from research I've done, unless the atmosphere is not breathable then it will always be blue, no matter what colour the sun is etc?

Is that really the case?

[JandenHale]

It could be the case, but I'm sure you could easily account for some other reason that would be plausible. Like maybe there's another gaseous element in that planet's atmosphere that gives the sky a different hue. Easy breezy. Or some non-toxic chemical they've been irresponsible with that did it. I think it can be done without much work.

[Warren_Paul]

Originally Posted by JandenHale

Or some non-toxic chemical they've been irresponsible with that did it.

That has potential, but then the story might come across too much like Serenity... chemical released into the atmosphere, changes colour of sky and turns people into zombies... yep, reivers!

[JandenHale]

Haha, yes! How could I forget? I'm sure you can think of something. Maybe you could experiment with how people perceive light, different spectrums, etc. I'm just spit-ballin here.

[Warren_Paul]

Yeah. I'm thinking the extra gas element yet still breathable will do the trick maybe

[juelz4sure]

One way to explain a different color sky could be caused by rings around the planet. For instance if you were to be on Saturn (I know it doesn't have a breathable atmosphere) but the rings make the sky a bright white i think. That's because the rings are majority ice and the sunlight reflecting off the rings are blindingly white. With that said you could use that type of an idea, and different parts of the planet have different color sky because of the angle the light reflects off the rings. just an idea.

[Warren_Paul]

oh yeah, that's certainly an interesting thought. I need somebody from Nasa!

The question is: Could earth have rings without causing problems with the atmosphere being breathable?

[juelz4sure]

I don't see why rings surrounding the planet would affect the gases in the atmosphere, Just because of the fact that space has no asteroids of any other large mass in space surrounding earth IE the moon has effect on the gases I would be more worried about the effect it might have on gravity though. I watch a lot of stuff on history science and discovery. I don't work for NASA but I slept at a Holiday INN

[Perpetual Man]

I agree with the others, I don't see any reason why the sky could not be a different colour. Moving aside from the rings, the sky changes colour now - red at night, red in the morning depending on cloud cover, sun position etc - and it can go through a range of colours in between, not to mention dark at night, grey during a storm.

A few years ago (might be a more than few) when there was the massive Icelandic volcanic eruption there were some spectacular colours in the sky, so it depends on how the light is filtered. (Like rings). If a massive dust cloud passed between the sun and us it might effect the light spectrum - of course there would be more elements at play - temperature, whether the cloud orbited the sun with us, or if it was only certain times of the year it was blocking the light etc.

[Abernovo]

Warren, I'm kind of glad you started this thread. I've been considering something along similar lines.

I'm not sure about blue being the only habitable sky colour, but from what I've picked up watching science programmes and some online research, the colour of the star a planet is orbiting would have an effect upon the light levels and light reflection on the planet's surface.

That may seem obvious, but it means, for instance, that on a planet orbiting a red star, plant leaves would appear almost black, as they would adapt to absorb pretty much all the radiation. On Earth, with a warmer, yellow star, they only need to absorb a portion of the light and the rest is 'reflected' off, which we see as green colour. Before any physicists jump on me, I know this is a very simplistic explanation, incorrectly termed.

Anyway, following this line of reasoning (plants appearing black), I'm assuming that planets orbiting cooler (orange,red) stars would have some very cold, heavily shaded areas, the rock hues would be different and, if there were seas, they might appear much darker, perhaps even purple-black. So, could the sky not be different as well? With a 'darker' star, there would probably be less planetary surface albedo, so (possibly) the sky would be darker, less 'solid' blue, perhaps making other planets in the system more visible during the day. That is speculation on my part.

Alternatively, on a hotter, brighter star, the sky might be even more 'solidly' blue, although the exact hue might be different from ours. Again, plant and surface colours could be different from what we are used to, so it might alter the perception of the world - like walking through a grey city makes everything look grey unless it's a very bright, sunny day.

Hmm, sorry, this has gone on much longer than I intended.

[Warren_Paul]

I really have no clue about astronomy and this stuff but from what I understand its all to do with a process called Rayleigh Scattering, where the atmosphere changes the colour of the light to what we perceive as blue.

Pulled from wiki

A portion of the light coming from the sun scatters off molecules and other small particles in the atmosphere. It is this scattered light that gives the sky its brightness and its color. As previously explained, Rayleigh scattering is inversely proportional to the fourth power of wavelength, so that shorter wavelength violet and blue light will scatter more than the longer wavelengths (yellow and especially red light). The resulting color, which appears like a pale blue, actually is a mixture of all the scattered colors, mainly blue and green. Violet, though strongly scattered, is a minor component of the solar spectrum and is less efficiently detected by the human eye. Conversely, glancing toward the sun, the colors that were not scattered away — the longer wavelengths such as red and yellow light — are directly visible, giving the sun itself a slightly yellowish hue. Viewed from outer space, however, the sky is black and the sun is white.

The reddening of sunlight is intensified when the sun is near the horizon, because the volume of air through which sunlight must pass is significantly greater than when the sun is high in the sky. The Rayleigh scattering effect is therefore increased, removing virtually all blue light from the direct path to the observer. The remaining unscattered light is mostly of a longer wavelength, and therefore appears to be orange.

Rayleigh scattering primarily occurs through light's interaction with air molecules. Or, from a purely macroscopic point of view, blue sky comes from microscopic density fluctuations, resulting from the random motion of molecules composing the air. A region of higher or lower density has a slightly different refractive index from the surrounding medium, and therefore it acts like a short-lived particle that can scatter light in random directions. Smaller regions fluctuate more than larger ones, and, since short wavelengths are disturbed by small regions more than longer wavelengths, they are scattered more.

Some of the scattering can also be from sulfate particles. For years after large Plinian eruptions, the blue cast of the sky is notably brightened due to the persistent sulfate load of the stratospheric gases.

In locations with little light pollution, the moonlit night sky is also blue, because moonlight is reflected sunlight, with a slightly lower color temperature due to the brownish color of the moon. The moonlit sky is not perceived as blue, however, because at low light levels human vision comes mainly from rod cells that do not produce any color perception.

Further reading that goes way over my head: Diffuse Sky Radiation

As I understand it, the sky changes colour at sunset because of the angle of the sun, but that phenomenon can only happen at that angle, not while the sun is passing through the sky.


EDIT:

The comments I'm finding are that the sun colour has no effect on the colour of the sky.

[Ursa major]

Originally Posted by Warren_Paul

So, basically, the colour of the sky is a function of:

• the wavelengths the star generates (in the sense that a wavelength that's absent can't be scattered);
• the angle of the star to the horizon;
• what molecules are in the air, and at what density (and variety of density?);
• the effects of Rayleigh Scattering; and
• the capabilities of the observer's vision system.

[Prefx]

I'm a senior physics undergraduate, so don't take my post as professional credibility, but I'll chime in by saying that if you want to keep things as realistic as possible, I'd suggest not emphasizing the neighborhood star unless it's a red giant (or blue dwarf - but that would require a lot of adaptations), in which case (depending on how close your planet is to said star) the alterations in sky colors will be less significant. Almost all stars appear white to the human eye.

Red giants and blue dwarfs are the exceptions. If you've ever looked at Betelgeuse in a telescope, it appears red-tinted. It's definitely something every sci-fi fan should experience!

Your best bet is to consider the atmosphere; for example, on Mars, the sky is (generally) brown or teal, but at sunrise and night the sky changes to a violet color with shades of orange/brown.

Just my opinion.

[Nik]

Don't forget 'vug', volcanic smog, can turn the sky red and the Moon green. It may be breathable, but it is unpleasant...

This suggests you'll also need to work out the world's plate tectonics, or some reason not to have such, like a much thicker crust. Latter may restrict routine activity to a couple of mega-shield volcanoes, as on Mars, with periodic (mega-year ;- ) 'overturns' gushing flood-basalt like Indian & Siberian Traps...

I've also seen suggestions about aerial plankton turning the sky purple. IMHO, that would need a significantly raised surface pressure to permit tiny balloons to work. If so, be careful with your hot drinks as the raised pressure increases boiling point...

[HuskerFanCO]

Hello all. First post, so hopefully it's helpful. I have been researching this as well, and here are two pretty good links I found regarding sky color and the potential effect on plant color.

http://epod.usra.edu/blog/2009/02/sk...exoearths.html

http://www.solstation.com/life/a-plants.htm
I believe this link has sub links to other scientific articles, but I haven't checked them out yet.

Of course, I am world building for a "science fantasy" universe, so the hard technical science is less important to me as long as it is reasonably plausible. You can always bend the "rules" unless you are writing "hard science fiction" where readers expect true, science-based explanations.


Cheers,
Troy