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A clear overhead sky is a medium blue.
Ignoring the effects of solid particulates, what color would the sky be if the atmosphere was 100% Nitrogen?
And 100% Oxygen?
And some change in the ratio between Oxygen and Nitrogen?
Could a sky hue change be expected if there was more Xenon, Neon or Carbon Dioxide in the air?
What would it take for Earth to have a green or indigo sky that still supported life as we know it?
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The more Nitrogen you have, them more 'Purpleish' the sky would seem, Nitrogen scaters light in the violet and near UV, where as oxygen scaters in blue and and Violet ranges.
Carbon Dioxide would scater in the IR spectrum, Neon a sort of redish orange, Xenon, an even lighter blue then Oxygen.
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I thought the blue color was due to the lambda^4 in the Rayleigh scattering equation. It's not clear to me that a different molecule would have any significant impact on this. If you want to get different colors you'll have to introduce dust particles, or at least something much larger than an individual molecule.Originally Posted by dgavin
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That's right; the atoms and molecules in air are all much, much smaller than the wavelengths of visible light (a few nm vs. hundreds of nm), so you get Rayleigh scattering almost no matter what the gas is. Mars's sky is brownish-yellow due to the extreme thinness of the atmosphere and all the dust that's constantly suspended in it.
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Hi,
besides the respective atmosphere's density and (if they are present) the amount and exact nature of dust particles contained in it, the situation as regards ambient lighting or illumination on the planet surface would of course be another, rather important factor. So because ambient light surely has an influence on how our eyes convey specific color impressions, the skies on two distinct planets sharing basically the same atmospheric configuration may, just owing to variations in terms of ambient light, still appear fairly different.
A fascinating topic, though there's no more I can contribute–perhaps save for this reference to an article by Stephen Inniss and Steve Bowers. Short, the essentials, yet clearly understandable.
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I'm pretty sure that the sky would be blue for most realistic compositions. Titan's clear upper atmosphere is blue in visible light photos. I also recall seeing an amazing Cassini close up of Saturn's limp that shows a sliver of blue above the clouds.
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Rayleigh scattering is also dependant on if the gas is sensitive to that frequency of light. If human had true vilolet receptors, the sky, as it is, would appear more vilolet then blue. However because we see violet as a interpolation of light triggering both blue and red receptors, we don't see the violet scatering, as it's over powered by our blue receptors. Violet is also scattered out earlier then blue light, so blue light scatering penetrates further into the atmosphere.
However a gas that absorbs and re-emits violet wavelenghts, such as nitirogen, would allow violet light to penetrate further before scattering. So actualy mixing different gasses in different amonts does effect what wavelenth scatters and deep the scattering penetrates. The wavelength that penetrates the most but still scatters, is the sky color you'll percieve.
It's a lot more complex then just the rayleigh model.
For example the green sky seen during sever thunderstorms, is caused by water in the air, causing an additional scattering of red light, which when mixed with the blue scatering gives you a green sky.
I've not researched what causes the rarer gold sky's we get in the mornings here, instead of typically pink dawns, but thats also related to water content in the air, and when you have thin high clouds presents, the gold sky can almost blaze on rare occasions for about a half hour in the mornings. I've seen it a few times now, and it always catches me off guard.
Order of Kilopi
Yet again - I'd like to know why you've asked WD40. You have so many unusual questions that you ask, and many you never come back to. What's you background motivation here - it would help all of us answer your questions more quickly, thoroughly and get to whatever you need.
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What would have been the likely difference from today, if any, in the Earth's sky's color 1 billion, 100 million, 1 million, 100,000 and even only 10,000 years ago?
Last edited by wd40; 2012-Jul-09 at 01:11 AM.
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I think the only significant difference would be when the earth atmosphere was mostly Methane some 4 billion years ago, in that case it probably had scattering that made it look more like Titan, a dull yellow/orange, then the blue of most other gasses. Basically a methane atmosphere would have the scattering of a 40-80 bar O2/C02 atmosphere.
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What of the upper Venusian atmosphere, about where the pressure is approximately 1 bar?
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I suspect that had the answer been that the sky was a different color a bazillion years ago, the followup question would have been something like "But how could chlorophyll have evolved when the sky was a different color?"
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No, The question would have been "how are chlorophyll and haemoglobin identical, except one has a magnesium atom at its center, and the other an iron one?"!
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The answer to that is simply that chlorophyll and haemoglobin structures are not identical - but they are similar - most likely because they both have a similar functional ability to coordinately bind a metal ion and the coordinating molecular structure was derived from available biochemical organic molecules.
The biochemical steps for the the formation of the chlorophyll molecule are nearly the same as for the production of the heme molecule (the basic porphyrin structure) and are thought to have been evolved from the earlier heme biosynthetic pathway.
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They aren't even similar, beyond incorporating porphyrin rings. Hemoglobin is a globular protein consisting of multiple units each containing a porphyrin ring, chlorophyll is a much simpler molecule consisting of a porphyrin ring with some attached groups, often a phytol chain.
Chlorophyll:
http://commons.wikimedia.org/wiki/Fi...ling_model.jpg
Hemoglobin:
http://chemistry.ewu.edu/jcorkill/biochem/fig_06_33.jpg
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So - given that two people have shown you how catastrophically wrong that question is...how or why did you come to place where you thought to ask it - what lead you to believe they're near identical?
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Hemoglobin
Chlorophyll
Order of Kilopi
That's heme b, not hemoglobin.
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"catastrophically wrong"
I would say that is a catastrophically wrong description of the wrongness. How about notching it down a touch?
This is aimed at everyone: I also have some problems with questioning the motivation of another member, in this case wd40, when they ask a completely legitimate question for S&T, about sky color. Frankly, questioning someone's motivations is a form of personal attack. "I'm just curious" is a perfectly legitimate reason to ask something. If you don't wish to answer their question, then don't. If you suspect a problem with what they post, Report it.
Order of Kilopi
This NASA artist's impression suggests that the sky above the clouds is blue;
http://www.universetoday.com/wp-cont...s-airplane.jpg
...however I seem to remember that one of the Russian probes took colorimeter readings on the way down, and the average colour received was greenish most of the way down - probably because of the sulphur content. I can't find a cite for that at the moment, though.
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