Post by Catsmate on Apr 28, 2014 22:39:56 GMT
Or rather what colour could it be on other planets? Hopefully these notes will be of use to GMs creating more realistic alien planets.
The colour of the sky (any sky) depends on the nature of the atmosphere, the source of illumination and the eye seeing it. Generally on Earth the light source is the sun (Sol to be formal), that big yellow ball of plasma 150Gm away. The main process determining the colour of the sky is Rayleigh scattering which is, in simple terms, the bouncing of light photons off the molecules of a gas. It's called "scattering" because, unlike reflection from a mirror or similar surface, the light bounces off the gas molecules at a wide range of angles.
Most of the light passing through the atmosphere doesn't actually scatter, but enough does so that any point in the atmosphere that's illuminated by the sun is effectively emitting a significant amount of light in all directions. So when you look into the sky (not directly at the sun), the light reaching your eyes has come from the sun via bouncing off one or more molecules of the atmosphere.
But why is it blue, when the sun appears a pale yellow colour? The answer lies in the physics of Rayleigh Scattering; the gist of which is that light of shorter wavelengths is more likely to scatter than light of longer wavelengths. Far more likely in fact. Long wavelength light is red, while short wavelength light is blue (an over-simplification but accurate enough). So when you look into the sky and see all that scattered light, most of it is blue.
Now you may be wondering along the lines of why isn't the sky purple; violet light is shorter in wavelength even than blue light, therefore it should be scattered more than blue light. Thus most of the light scattered from the atmosphere should be violet. This is actually quite correct, at least in that violet light is scattered more than blue. But there are a couple of other factors.
Firstly the light source (the aforementioned yellow plasma ball) does not emit equal intensities of light across the visible spectrum. In fact the intensity of sunlight peaks in the yellow region; there's a bit less green and orange, far less blue and red, and very little violet by comparison. The amount of violet in sunlight is so little that, even though more strongly scattered, its intensity is roughly equal to that of the scattered blue light.
Secondly there's our light sensor; the human eye. This is much more sensitive to blue light than violet light; given equal intensities across this part of the colour spectrum, we tend to see the result as just blue, rather than a violet/blue/indigo mix.
At sunset and sunrise sunlight must pass through a much thicker layer of atmosphere to reach your eyes, because of the low angle of incidence. Thus incoming light is more likely to scatter, and in fact is more prone to scatter multiple times.
The result of this is that the blue light, which remember tends to scatter more frequently, ends up scattered by greater accumulated angles than red light. So when looking at a sunset, most of the light near the sun is this minimally scattered red light, and as you look further from the sun it fades through orange and yellow back to blue.
Lots of CO2 or better still (for a certain meaning of better anyway) large molecules such as sulphuric acid would lighten the sky colour even more, giving a pale yellow like that seen in some photos from the Martian surface.
So what about other planets?
A duplicate of Earth but with argon in place of nitrogen would have a darker blue/purple sky.
A Venus type planet has an extremely thick, dense, atmosphere full of large molecules, like that sulphuric acid I mentioned, so the sky is a dark orange-red (probably).
On Mars the atmosphere is very thin and Rayleigh scattering is of limited effect in determining sky colour (dust is more significant). Overall it's orange-red shading to scarlet.
In general for extra-terrestrial planets:
If you want a bright green sky, select a red, cooler, star, a planet with Terrestrial or lower gravity (to give a thiner atmosphere) composed of a somewhat higher proportion of oxygen (assuming you want it human breathable) and a higher proportion of argon (most lighter gases are either too reactive or won't be held in by the gravity).
If you want a pale yellow sky, but a human breathable atmosphere, you want a planet with a bluer star, a thicker atmosphere rich in heavier gases, but not ones inimical to human life so no sulphuric acid and CO2 should to be 1% at most, perhaps xenon or sulphur hexafluoride. Somewhat lower in oxygen, to compensate for the higher pressure.
The colour of the sky (any sky) depends on the nature of the atmosphere, the source of illumination and the eye seeing it. Generally on Earth the light source is the sun (Sol to be formal), that big yellow ball of plasma 150Gm away. The main process determining the colour of the sky is Rayleigh scattering which is, in simple terms, the bouncing of light photons off the molecules of a gas. It's called "scattering" because, unlike reflection from a mirror or similar surface, the light bounces off the gas molecules at a wide range of angles.
Most of the light passing through the atmosphere doesn't actually scatter, but enough does so that any point in the atmosphere that's illuminated by the sun is effectively emitting a significant amount of light in all directions. So when you look into the sky (not directly at the sun), the light reaching your eyes has come from the sun via bouncing off one or more molecules of the atmosphere.
- This is why the daytime sky is bright all around. This is also why might can get into places, like under your bed, when there's no direct path.
- Have a look at the Apollo pictures from the lunar surface sometime and see how stark the shadows are, and how abrupt the divide between light and shadow is.
But why is it blue, when the sun appears a pale yellow colour? The answer lies in the physics of Rayleigh Scattering; the gist of which is that light of shorter wavelengths is more likely to scatter than light of longer wavelengths. Far more likely in fact. Long wavelength light is red, while short wavelength light is blue (an over-simplification but accurate enough). So when you look into the sky and see all that scattered light, most of it is blue.
Now you may be wondering along the lines of why isn't the sky purple; violet light is shorter in wavelength even than blue light, therefore it should be scattered more than blue light. Thus most of the light scattered from the atmosphere should be violet. This is actually quite correct, at least in that violet light is scattered more than blue. But there are a couple of other factors.
Firstly the light source (the aforementioned yellow plasma ball) does not emit equal intensities of light across the visible spectrum. In fact the intensity of sunlight peaks in the yellow region; there's a bit less green and orange, far less blue and red, and very little violet by comparison. The amount of violet in sunlight is so little that, even though more strongly scattered, its intensity is roughly equal to that of the scattered blue light.
Secondly there's our light sensor; the human eye. This is much more sensitive to blue light than violet light; given equal intensities across this part of the colour spectrum, we tend to see the result as just blue, rather than a violet/blue/indigo mix.
At sunset and sunrise sunlight must pass through a much thicker layer of atmosphere to reach your eyes, because of the low angle of incidence. Thus incoming light is more likely to scatter, and in fact is more prone to scatter multiple times.
The result of this is that the blue light, which remember tends to scatter more frequently, ends up scattered by greater accumulated angles than red light. So when looking at a sunset, most of the light near the sun is this minimally scattered red light, and as you look further from the sun it fades through orange and yellow back to blue.
- This creates the classic red/gold sunsets. Pretty.
- Massive dust additions to the atmosphere, such as the eruptions of volcanoes like Krakatoa or Pinatubo can have an effect on sunsets globally that lasts years. Of course if they're large enough (Tambora and the "Year Without a Summer" for example) they can significantly effect global climate.
- Pretty sunsets, hundreds of thousands starve and Frankenstein is written. History is weird sometimes.
- Bigger molecules mean more scattering of the incoming light (for CO2 the scattering is ~100x as great as for nitrogen), and this is more pronounced in the longer wavelengths, so the sky would be lighter, greenish blue. Turquoise perhaps.
Lots of CO2 or better still (for a certain meaning of better anyway) large molecules such as sulphuric acid would lighten the sky colour even more, giving a pale yellow like that seen in some photos from the Martian surface.
- Pretty but a little hard on humans, not to mention plants, animals, buildings.......
So what about other planets?
A duplicate of Earth but with argon in place of nitrogen would have a darker blue/purple sky.
- Though it would also have some serious problems, nitrogen is vital for biological processes like the formation of proteins
A Venus type planet has an extremely thick, dense, atmosphere full of large molecules, like that sulphuric acid I mentioned, so the sky is a dark orange-red (probably).
- There's not a lot of actual data, probes tend to dissolve.
- Or do they? Perhaps it's just the Venusians maintaining their privacy.
On Mars the atmosphere is very thin and Rayleigh scattering is of limited effect in determining sky colour (dust is more significant). Overall it's orange-red shading to scarlet.
- Even if a planet has a nominally breathable atmosphere dust can be dangerous, to human health and to equipment, perhaps necessitating filter masks and sealed buildings. Some dusts, like small amounts of beryllium, can have more subtle effects on human health. Interesting possibilities for exploring new worlds.
In general for extra-terrestrial planets:
- thinner atmosphere and/or lighter atmospheric gases cause a darker, bluer sky
- conversely a thicker atmosphere or heavier atmospheric gases cause a lighter, redder sky
- a redder or bluer star pushes the sky colour in that direction
- pollutants or dust cause a red tinge, overlaid on the principal colour
If you want a bright green sky, select a red, cooler, star, a planet with Terrestrial or lower gravity (to give a thiner atmosphere) composed of a somewhat higher proportion of oxygen (assuming you want it human breathable) and a higher proportion of argon (most lighter gases are either too reactive or won't be held in by the gravity).
If you want a pale yellow sky, but a human breathable atmosphere, you want a planet with a bluer star, a thicker atmosphere rich in heavier gases, but not ones inimical to human life so no sulphuric acid and CO2 should to be 1% at most, perhaps xenon or sulphur hexafluoride. Somewhat lower in oxygen, to compensate for the higher pressure.
- A xenon rich atmosphere would have some interesting effects; it can pool due to it's density and accumulate to hazardous levels (breathing 80% xenon with 20% oxygen causes rapid unconsciousness, it has been used in surgical anaesthesia). It also lowers the pitch of the voice, the opposite effect to helium. Chemically it's almost entirely inert.
- Sulphur hexafluoride is also chemically inert but less so than xenon, so you'd need to account for it's continued presence in the atmosphere (or just mutter about "primordial vulcanism" or "strange microbes" and ignore it). Unlike xenon it can be broken down by electrical discharges, forming a nasty poisonous gas disulphur decafluoride (S2F10). It's also a greenhouse gas almost twenty five thousand times as potent as carbon dioxide so a planet with an atmosphere rich in SF6 will tend to be hot, especially given that blue stars tend to be more energetic.