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OtisTheWondernerd
2007-Jan-19, 07:52 AM
One sci-fi staple that I've never come across a discussion of is the odd-colored alien atmosphere. Whether it's green, or purple, or orange, there are countless worlds in fiction with an unusual sky color - which is, of course, perfectly breathable by humans.

I'm curious to find out A) what sort of atmospheric compositions could reflect a uniformly non-blue color, and B) whether these atmospheres would be human-friendly. To make life easier for all of us, let's assume that the world in question is identical to Earth in all other respects, and orbits a likewise identical star.

Thanks in advance! :)

01101001
2007-Jan-19, 08:53 AM
I'm curious to find out A) what sort of atmospheric compositions could reflect a uniformly non-blue color, and B) whether these atmospheres would be human-friendly. To make life easier for all of us, let's assume that the world in question is identical to Earth in all other respects, and orbits a likewise identical star.

I'm not sure you can draw any definite conclusions about the human-friendliness of various sky colors. If the illumination source is the same, at least that variable... doesn't vary. Dusts, droplets, solutes, vapors, could make some difference and be of no health consequence.

Orion's Arm has a little about alien atmosphere colors: The Sky on Alien Worlds (http://www.orionsarm.com/whitepapers/sky_on_alien_worlds.html)

sarongsong
2007-Jan-19, 09:17 AM
Interesting color article from Space.com:
June 25, 2002
Earlier this year two astronomers from Johns Hopkins University announced they had determined the collective color of the cosmos...science reporters and editors recognized the street value of the story and fell for it like paparazzi on a J. Lo sighting... Coloring the Universe: Why Reality is a Gray Area in Astronomy (http://www.space.com/scienceastronomy/astronomy/color_universe_020625-1.html)

grant hutchison
2007-Jan-19, 12:39 PM
We had a very long thread on this topic some time ago, but for the life of me I can't find it. Let's see if I can summarize.
Load the sky with coloured particulates, and you can have any colour you want.
If your atmosphere is composed of colourless gases and minimal particulates, then its colour will come from Rayleigh scattering. That gives a sequence of sky colours as atmospheric density increases:
Blue horizon, black or dark blue zenith (thinner than Earth's atmosphere)
Blue zenith, pale or white horizon (like Earth)
Horizon whiteness spreading towards the zenith, and "closing" to give a completely white sky somewhere around 10 atmospheres.
Beyond that, the sky progresses through a set of sunset colours: pale yellow, orange, red, dark red, utter darkness. (The "Belt of Venus", which rises in the east as the sun sets, gives an impression of what these shades might be like.)
There's also a possibility of a very delicate pale green in the transition from white to pale yellow: some reliable observers say they've detected such a shade in the Belt of Venus, but I never have. These dense-atmosphere sky colours would be very dependent on the height of the sun: you might have a pale yellow sky at mid-day which turned orange by mid-afternoon, for instance.

The trouble with the exotic sky colours is they all require atmospheres denser than 10 atmospheres. An Earth-type mixture of nitrogen and oxygen would be unsafe to breathe at that pressure: you'd either suffer oxygen toxicity or nitrogen narcosis or both.

Grant Hutchison

Romanus
2007-Jan-19, 02:59 PM
Scientists have theorized that before Earth's atmosphere had a lot of oxygen, that the sky was brownish or tawny in color, a result of a reducing environment.

The only condition I could imagine a differently-colored atmosphere being breathable would be around a red dwarf, in which there would be much less blue light for any atmosphere to scatter.

mb_webguy
2009-May-03, 07:19 AM
The only condition I could imagine a differently-colored atmosphere being breathable would be around a red dwarf, in which there would be much less blue light for any atmosphere to scatter.

Suppose Mars had an atmosphere with a composition identical to Earth's. Suppose also that it had the same pressure at sea level as Earth. Because of the lower gravity, the atmosphere would necessarily thicker (as in height) in order to create that pressure. Wouldn't Rayleigh scattering cause the sky to be similar in color to the sunsets of Earth, as the sunlight is passing through more atmosphere to reach the surface? And since it is the same composition and pressure as Earth's at the surface, it would be just as breathable.

What I'm curious about is differences in composition and density. For example, suppose Earth's atmosphere (or the hypothetical atmosphere of Mars in the above example) contained a significantly higher percentage of some other noble gas, such as krypton, in place of some nitrogen. For example, suppose instead of the 78% nitrogen of the actual atmosphere and 1ppm krypton, that it was instead more like 39% nitrogen and 39% krypton. Krypton is colorless, odorless, and inert, just like nitrogen, but is considerably more dense. With this composition, the atmosphere would be about twice as dense as Earth's actual atmosphere, but (assuming similar pressure) completely breathable. Would this change in atmospheric density affect sky color?

eburacum45
2009-May-03, 09:36 AM
Twice as dense doesn't seem to be enough of an increase in density to cause a substantial change in sky colour, except that the pale tint at the horizon would reach further up the sky.

By about 10 atmospheres you would see a greenish tint, if I am interpreting figure 5 of Craig Bohren's paper correctly, here
http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf
but the white horizon tint would reach pretty high on such a world.

chornedsnorkack
2009-May-03, 10:28 AM
Suppose Mars had an atmosphere with a composition identical to Earth's. Suppose also that it had the same pressure at sea level as Earth. Because of the lower gravity, the atmosphere would necessarily thicker (as in height) in order to create that pressure. Wouldn't Rayleigh scattering cause the sky to be similar in color to the sunsets of Earth, as the sunlight is passing through more atmosphere to reach the surface? And since it is the same composition and pressure as Earth's at the surface, it would be just as breathable.

Probably yes. Thicker atmosphere basically means that the near-horizon/sunset sky colour goes higher up in the sky. And vice versa.


What I'm curious about is differences in composition and density. For example, suppose Earth's atmosphere (or the hypothetical atmosphere of Mars in the above example) contained a significantly higher percentage of some other noble gas, such as krypton, in place of some nitrogen. For example, suppose instead of the 78% nitrogen of the actual atmosphere and 1ppm krypton, that it was instead more like 39% nitrogen and 39% krypton. Krypton is colorless, odorless, and inert, just like nitrogen, but is considerably more dense. With this composition, the atmosphere would be about twice as dense as Earth's actual atmosphere, but (assuming similar pressure) completely breathable. Would this change in atmospheric density affect sky color?

No, not completely breathable. Krypton is much stronger narcotic than nitrogen. I think that it is a slightly stronger Rayleigh scatterer, too. Someone found the numbers for those.

grant hutchison
2009-May-03, 12:37 PM
No, not completely breathable. Krypton is much stronger narcotic than nitrogen. I think that it is a slightly stronger Rayleigh scatterer, too. Someone found the numbers for those.If you're referring to the NASA Technical Note I referenced on another thread, it didn't deal with the less common noble gases.
Krypton is listed with a narcotic potential about seven times that of nitrogen in the inert diving manuals. So if we replaced half of our (1 bar) atmosphere's nitrogen with krypton, we'd have the narcotic equivalent of 4-bar air: the equivalent of a 30-metre dive, which is at the threshold for significant impairment. Breathing that mixture, you'd already feel like you had a couple of martinis on board, according to Martini's famous "Law".

Grant Hutchison

mb_webguy
2009-May-03, 08:58 PM
By about 10 atmospheres you would see a greenish tint, if I am interpreting figure 5 of Craig Bohren's paper correctly, here
http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf
but the white horizon tint would reach pretty high on such a world. Thanks for that link! That's a very interesting and informative article. However, I'm not quite sure about your interpretation of that figure, though I agree with your conclusion. The figure shows comparitive thickness of atmosphere, not density. But yes, on further thought, twice the density is probably not enough to make a noticable difference in sky color.

If you're referring to the NASA Technical Note I referenced on another thread, it didn't deal with the less common noble gases.
Krypton is listed with a narcotic potential about seven times that of nitrogen in the inert diving manuals. So if we replaced half of our (1 bar) atmosphere's nitrogen with krypton, we'd have the narcotic equivalent of 4-bar air: the equivalent of a 30-metre dive, which is at the threshold for significant impairment. Breathing that mixture, you'd already feel like you had a couple of martinis on board, according to Martini's famous "Law".

Grant Hutchison
When I wrote my first post, I was going by this blurb from the Wikipedia article on Krypton. I found multiple other sources, all using the mix of 50% krypton / 50% air.
Krypton has a narcotic potency seven times greater than air, so breathing a gas containing 50% krypton and 50% air would cause narcosis similar to breathing air at four times atmospheric pressure. This would be comparable to scuba diving at a depth of 30 m (100 ft) (see nitrogen narcosis) and potentially could affect anyone breathing it.
That's why I chose to replace half of the nitrogen in my hypothetical situation with krpyton rather than simply making krypton half the total composition of air. I was assuming that at 38%, any narcosis would be less than the equivalent to diving to less than 30m, and therefore result in at most mild euphoria.

My interest in this, btw, other than my natural curiosity, is that I'm in the process of writing a novel -- actually a series of short stories -- which would involve a small planet (roughly the size of Mars) with an engineered atmosphere, which for certain specific reasons is intentionally more dense than Earth's. I've always hated sci-fi stories with lousy science, so I'm trying to be as accurate as possible. My own background is in computer science, so I've had to do a bit of research outside of my specialty. Most of the issues involved with life on a small planet with a slightly more dense atmosphere are pretty straightforward, but accurately determining sky color has actually been rather difficult. There's a lot of contradictory information floating around, even among people who should know better.

grant hutchison
2009-May-04, 12:11 PM
That's why I chose to replace half of the nitrogen in my hypothetical situation with krpyton rather than simply making krypton half the total composition of air. I was assuming that at 38%, any narcosis would be less than the equivalent to diving to less than 30m, and therefore result in at most mild euphoria.Take the narcotic potential of sea-level atmospheric nitrogen as "1 narc". Replace half with krypton: now you have 3.5 narcs of krypton plus half a narc of nitrogen. That's the equivalent of four times the original nitrogen, which takes you right to the 30m depth, breathing four-bar air.
There's a continuous progression through the levels of general anaesthesia with increasing depth, and different people respond differently, just as they do to other general anaesthetics. Diving with air, some people get a significant hit at 25m depth, others can get down to 35m without a problem. So if your atmosphere were an over-the-counter medicine, it would require a "do not drink alcohol, drive or operate dangerous machinery" advisory, and it certainly has the potential to knock a few people (especially the elderly) entirely off their mental perch.

Grant Hutchison

Jeff Root
2009-May-04, 01:26 PM
I noticed on a plane flight in February 2002 that the sky immediately
above the level of the white clouds was green. It was a definite band
of green between the clouds and the blue sky above. I didn't see it
a week later on the flight back.

-- Jeff, in Minneapolis

mb_webguy
2009-May-04, 02:15 PM
Take the narcotic potential of sea-level atmospheric nitrogen as "1 narc". Replace half with krypton: now you have 3.5 narcs of krypton plus half a narc of nitrogen. That's the equivalent of four times the original nitrogen, which takes you right to the 30m depth, breathing four-bar air.
[...]
So if your atmosphere were an over-the-counter medicine, it would require a "do not drink alcohol, drive or operate dangerous machinery" advisory, and it certainly has the potential to knock a few people (especially the elderly) entirely off their mental perch.

Grant Hutchison

Well... That would certainly put a bit different spin on the story... :) Thanks.

chornedsnorkack
2009-May-04, 02:44 PM
Does Earth atmosphere have any collision-induced absorption lines?

Is the blue colour of oxygen caused only by oxygen/oxygen collisions, or do oxygen/inert collisions contribute?

Jeff Root
2009-May-04, 03:59 PM
This is different.

:)

-- Jeff, in Minneapolis

George
2009-May-04, 04:24 PM
Does Earth atmosphere have any collision-induced absorption lines? It does have absorption lines throughout the spectrum. Ozone is especially handy as it "absorbs" UV.

Speaking of ozone, it greatly contributes to the blue sky that we see overhead during a sunrise or sunset, otherwise the overhead sky at this time would be more yellow or redish.


Is the blue colour of oxygen caused only by oxygen/oxygen collisions, or do oxygen/inert collisions contribute? Oxygen molecules are not easy to paint. ;) They don't come blue, but when white light encounters them, the photons in the blue end of the spectrum will be scattered away from the orginal beam direction, and the red end portion will be favored in the original beam direction. Blue light will scatter about 5x to 9x more than the reds (depending on which wavelengths you select for those colors).

grant hutchison
2009-May-04, 04:59 PM
Oxygen molecules are not easy to paint. ;) They don't come blue ...Diatomic oxygen is blue, though only when very compressed or in the liquid state. I think this is what chornedsnorkack is referring to. And it does appear (http://www.rsbs.anu.edu.au/O2/O2_1_ ElectronicConfig.htm) that the blue colour requires two oxygen molecules in close proximity.

Grant Hutchison

chornedsnorkack
2009-May-04, 06:10 PM
Diatomic oxygen is blue, though only when very compressed or in the liquid state. I think this is what chornedsnorkack is referring to. And it does appear (http://www.rsbs.anu.edu.au/O2/O2_1_ ElectronicConfig.htm) that the blue colour requires two oxygen molecules in close proximity.

I see.

The full atmosphere would be equivalent of roughly 10 m of liquid air, or perhaps 2 m liquid oxygen. Liquid oxygen is rather distinctly blue even in a test tube (1 cm or so across).

How colourful is liquid air? What is as blue as 1 cm of pure liquid oxygen - 5 cm of liquid air or 25 cm of liquid air?

grant hutchison
2009-May-04, 06:33 PM
Holleman-Wiberg (Inorganic Chemistry) reports that liquid air is almost colourless, and becomes bluer on standing in an open vessel, as the nitrogen boils off preferentially.

Grant Hutchison

Magnetar
2009-May-04, 09:57 PM
http://www.orionsarm.com/whitepapers/sky_on_alien_worlds.html

Different types of stars can alter the color of the atmosphere, due to the differences in wavelengths of light said stars emit. According to that article anyway.

George
2009-May-05, 02:22 AM
Diatomic oxygen is blue, though only when very compressed or in the liquid state. I think this is what chornedsnorkack is referring to. And it does appear (http://www.rsbs.anu.edu.au/O2/O2_1_ ElectronicConfig.htm) that the blue colour requires two oxygen molecules in close proximity. I doubt it is applicable to the sky's color, which is what I thought your earlier post was addressing. Liquid water is also blue, but look at the color of water in a vapor state... it is white (Mie Scattering). I vaguelly recall someone trying hard to claim the air was indeed blue in color (non-Rayleigh scattering), and I argued with him in another forum as well.

There is a great book, "Why the Sky is Blue", which you would enjoy as it well written and full of more details than one might expect.

eburacum45
2009-May-05, 06:31 AM
http://www.orionsarm.com/whitepapers/sky_on_alien_worlds.html

Different types of stars can alter the color of the atmosphere, due to the differences in wavelengths of light said stars emit. According to that article anyway.
As one of the authors of that page, I would point out it is attached to a fiction site, so although we have tried our best, it is not peer reviewed science. We have had some suggestions on how to improve and clarify that page from members of this forum, but so far we haven't gotten round to implementing those changes.

Unfortunately the table which showed the possible effects of different star colours on sky colours was on a different site, and is no longer available. But if I recall correctly the star colour had little effect except for planets orbiting quite cool red dwarf stars.

galacsi
2009-May-05, 10:31 AM
Take the narcotic potential of sea-level atmospheric nitrogen as "1 narc". Replace half with krypton: now you have 3.5 narcs of krypton plus half a narc of nitrogen. That's the equivalent of four times the original nitrogen, which takes you right to the 30m depth, breathing four-bar air.
There's a continuous progression through the levels of general anaesthesia with increasing depth, and different people respond differently, just as they do to other general anaesthetics. Diving with air, some people get a significant hit at 25m depth, others can get down to 35m without a problem. So if your atmosphere were an over-the-counter medicine, it would require a "do not drink alcohol, drive or operate dangerous machinery" advisory, and it certainly has the potential to knock a few people (especially the elderly) entirely off their mental perch.

Grant Hutchison

I am sure life can adapt to this , even human beings. On Earth we have mammals who currently dive several hundreds of meter deep and all kind of fishes and other beasties live on the ocean floor. So you can put all the krypton you like in your SF mixture !

Galacsi.

grant hutchison
2009-May-05, 12:12 PM
On Earth we have mammals who currently dive several hundreds of meter deep and all kind of fishes and other beasties live on the ocean floor.Of course, the diving mammals take just a lung-full of nitrogen with them, so they're not susceptible to either nitrogen narcosis on the way down or the bends on the way up: there's no way for them to push their tissue nitrogen levels much higher than the sea-level norm.
Certainly deep-sea creatures have to adapt to high pressures in many ways. But how much dissolved nitrogen is there in abyssal water? I don't know, but if I were forced to guess, I'd think the partial pressure is low rather than high. Anyone got a good reference?

Grant Hutchison

chornedsnorkack
2009-May-05, 02:03 PM
Of course, the diving mammals take just a lung-full of nitrogen with them, so they're not susceptible to either nitrogen narcosis on the way down or the bends on the way up: there's no way for them to push their tissue nitrogen levels much higher than the sea-level norm.
Certainly deep-sea creatures have to adapt to high pressures in many ways. But how much dissolved nitrogen is there in abyssal water? I don't know, but if I were forced to guess, I'd think the partial pressure is low rather than high. Anyone got a good reference?


My guess is that the partial pressure is the same as in atmosphere. The nitrogen is dissolved at surface, and when the water sinks nothing will add or remove any.

grant hutchison
2009-May-05, 02:16 PM
My guess is that the partial pressure is the same as in atmosphere. The nitrogen is dissolved at surface, and when the water sinks nothing will add or remove any.I was guessing that where there's nitrogen and a need to build protein, there would be nitrogen-fixing bacteria. But I don't know.

Grant Hutchison