Atmosphere colors

[Enio]

Rassilon,
Some planets you do have purple atmospheres. What gases these atmospheres have? Oxigen? These planets are in Baramal and some systems. Also, the planets in Doctor Who Universe have purple and pink atmospheres.

[Rassilon]

My guess was methane for the gas giant and regular ovygen for the earth type...Color due to the star color...

[Eburacum]

My colleague Stephen Inness, at OA, has written this essay on sky colour,, if it is of any help;
it is illustrated using Celestia, of course.

http://www.orionsarm.com/whitepapers/sk ... orlds.html

[Guest]

My colleague Stephen Inness, at OA, has written this essay on sky colour,, if it is of any help

Nice. I think he goes astray a little with the green/yellow/red skies from Rayleigh scattering as the atmosphere thickens, however.
Here's what goes on:
A lot of the blue sky light you see comes from ~10km away ... you can see it being scattered between your eye and distant hills, for instance. Red light is also being scattered, but only 1/16 as much, so the blue predominates.
The same thing is happening in a parcel of air 40km away, with one difference ... blue light scattered towards your eye at that distance is likely to be scattered again before it reaches your eye. The small amount of red light, however, is so poorly scattered that it is likely to reach your eye directly.
So you can imagine there's a layered effect going on ... blue light generally comes from close to, green light from farther away, red light from the far distance.
The Earth's atmosphere is sufficiently thick that blue light predominates if you look straight up ... the air is just too thin at high altitude to introduce a red component. Looking out horizontally, though, you "see" >30 times as much air. All wavelengths are being scattered towards your eye from somewhere out there, so the horizon looks white, even in the absence of Mie scattering.
To get a sky coloured other than blue, you need to somehow eliminate the nearby "blue air". This happens if dense cloud or an eclipse shadows the air nearby, allowing you to look a long way out to sunlit sky on the horizon. Under these circumstance you can see the colour of distant air, and you'll often see red, or a desaturated greenish yellow (red+green) coming to your eye from afar.
But with open air and no shadows all that will happen if you make the atmosphere denser is that the whiteness of the horizon will close upwards towards the zenith. As the air gets less dense, the zenith will get darker and the horizon will become bluer.
No other colours from Rayleigh alone, I'm afraid.

Grant

[Eburacum]

Thabnks, Grant; I'll pass that on.

One interesting fact that used to be on Les Cowley's excellent Sundog site (but isn't there any more) is that the blue colour in a clear sky is caused by Rayleigh scattering int he thin upper atmosphere; lower down the scattering was negated by destructive interference (speaking from memory)
I wonder why he removed it...

So this could be another reason for dense, dust free atmospheres to have Rayleigh scattering only at the top of the atmosphere making the sky blue on many worlds.
.

[granthutchison]

One interesting fact that used to be on Les Cowley's excellent Sundog site (but isn't there any more) is that the blue colour in a clear sky is caused by Rayleigh scattering int he thin upper atmosphere; lower down the scattering was negated by destructive interference (speaking from memory)

Odd. Just looking at a distant horizon shows us that blue light is also scattered near the surface of the Earth, between our eyes and distant mountains.

Grant

[eburacum45]

Just looking at a distant horizon shows us that blue light is also scattered near the surface of the Earth, between our eyes and distant mountains.

I think that might be Rayleigh scattering due to small particles and droplets; the scattering in the upper air occurs between widely spaced air molecules.

I have a mind to write to Les Cowley and ask his opinion.

[granthutchison]

I think that might be Rayleigh scattering due to small particles and droplets; the scattering in the upper air occurs between widely spaced air molecules.

Rayleigh scattering is, almost by definition, at the molecular level, since the particles have to be of a size comparable to the wavelength of visible light. Most particulate matter is too big for Rayleigh scattering - it scatters all wavelengths equally, and so introduces a white haze. So it's the blueness of distant objects seen on a clear day that lets us know molecular-level scattering is happening in the intervening space.
But for sure, in many parts of the world, haze predominates ... and may even introduce colour because it absorbs certain wavelengths preferentially (like the orange haze of Titan).

Grant

PS: To see the contrast between Rayleigh and Mie (particulate) scattering, find a smoker. Cigarette smoke is pretty much molecule-sized ... just a cluster of carbon atoms, and so it looks blue in transmitted light. But if you inhale the smoke and breathe it out again, it turns white. The tiny droplet of condensation on each carbon nucleus makes it too big for Rayleigh scattering and turns the smoke into "haze".

[granthutchison]

In case you don't believe me (and why should you?) a quotation from Color and Light in Nature, by David Lynch & William Livingston (an atmospheric physicist and an astronomer), 2nd edition, 2001, ISBN 0 521 77504 3.

Distant mountains look bluish ... This blue veil is called airlight. Airlight is most evident when the sky is clear and is seen against distant objects.
Airlight is simply sunlight that is scattered by air molecules between us and the mountain.
...
Haze can masquerade as airlight. Unlike true airlight which is blue, haze appears grey, white or brown.

Grant

[Enio]

I've made a planet with a radius of 8635 Km with 2x the gravity on Earth and a red atmosphere with a pressure of 10x the Earth.

[Cham]

I've made a planet with a radius of 8635 Km with 2x the gravity on Earth and a red atmosphere with a pressure of 10x the Earth.

Good for you !

[eburacum45]

I wrote to Les Cowley, owner of the Sundog site
http://www.sundog.clara.co.uk/atoptics/phenom.htm
at last;

I remembered something on your site about
Rayleigh scattering being much more important in the thinner upper
atmosphere and the effects being lessened by destructive interference;

your site no longer mentions this as far as I can see, and it seems to
conflict with the phenomenon of 'airlight'; this is definitely blue and
apparently caused by Rayleigh scattering.

Does any of this have anything to do with the Einstein/Smouluchowski
scattering that you formerly mentioned?

Hoping you can clear this up...

his answer;

Yes, I recently removed the note about E-S scattering because
it was causing some confusion. When I get more time I want to put in a clearer text.

In the dense lower atmosphere, Rayleigh scattering by air molecules themselves
still takes place and will cause direct sunlight to be reddened. However, there is
not significant sideways scattered blue light from molecules. This is because
molecular scatterers are much closer together than the wave crests of visible light
and there is signifcant destructive interference in the transverse direction. A
quirk of geometry is that destructive interference does not occur in the forward
direction.

Another source of scattering is transient density fluctuations in the air arising from
molecular motions and collisions. These are on a larger scale than the separation
between individual molecules and they _do_ produce a transversely scattered
blue excess in the lower atmosphere. This is E-S scattering.

But to some extent this is pedantry because both types of scattering follow the
same angular and wavelength dependence. Thus to simulate airlight and sky
colours Rayleigh's equations are fully adequate.

Have I just added to the confusion .

Les

[granthutchison]

Ah, thanks for posting that, eburacum.
I love these little tweaks of irrelevant complexity.

Grant

[Velconti]

Hi,

my name is Marko and this is my first post at this form. I am creating a fantasy world called "Conjunja" in german language.

I have a weird solar system with three centered suns and a bulk of planets that circle around each other while orbiting the suns. on of these planets is called Conjunja.

I have programmed an own calendar and a "Planetary" that shows the actual sky and which of the planets and suns are visible that moment.

Take a look at http://www.conjunja.com/conjunja.php

By clicking <-Ereignis and Ereignis-> you can jump to other dates and get a glimpse of really strange colors.

PS: I will translate Conjunja into english in the nearer future. Anyone interested in contributing ?

m v @ vel com.d e

[Rassilon]

Well Ive always said that with gasses alien to us and our knowledge any sort of atmosphere color could be possible. Just not quite as likely I suppose...More earth type planets would resemble a blue sky than pink....But then whos to say....Thats what the imagination is for....