Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 05/08/2013 22:30:01
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Jeff Saul asked the Naked Scientists:
I am a physics and astronomy lecturer at the University of New Mexico and students in my introductory astronomy class have the following question. Could a planet have a green atmosphere and if so, what gases would it consist of?
I am a longtime listener (http://www.thenakedscientists.com/HTML/podcasts/) and download your podcast (http://www.thenakedscientists.com/HTML/podcasts/) each week to listen to in the car.
Thanks and best wishes,
Jeff Saul
What do you think?
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Damned if I know? what does the blueness we perceive rest on? Scattering of light in the atmosphere? One frequency becoming abundant? But that would also have a direct coupling to the chemistry, wouldn't it?
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ahh, you said it :) sorry.
Well, if it gotta be green it gotta be green as we say. It should be a combination of what star you have and the composition of gases, I guess? So, what gases would give green using our sun?
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Hmmm...
So, the first question would be why the sky is blue.
Looking at the solar light absorption spectrum.
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The peak solar light reaching the upper atmosphere is between 425nm and 500nm, in the middle of the blue spectrum. The sky, however, isn't completely transparent to blue, and some of the blue is absorbed and scattered, and thus less gets to the lower atmosphere. Is this caused by the ozone?
So...
If our sun was significantly cooler, the whole spectrum would be shifted to the right, and our sky would be green, without requiring any changes on Earth.
If there was a gas in the atmosphere that strongly absorbed blue, so little or no blue light would get to the surface, then our sky would also appear green. For example, Hydrogen has a strong absorption at 486nm, in the middle of the blue spectrum. If we had a Nitrogen/Hydrogen atmosphere, then our sky would be green (note, an Oxygen/Hydrogen atmosphere would be unstable). However, one might still need the ozone layer to give the nice color of the sky.
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Perhaps chlorine gas? It is greenish in color due to light absorption as opposed to blue sky light scattering, not sure what it would look like from a planet surface, sunset (starset?) should look pretty green.
A side note, on the great plains of Kansas we often get "green boomers", at the front of a strong thunderstorm the clouds will appear green. I've been told this is due to light scattering by hail/ice particles but have not researched this to confirm. This came to mind as we just had one of these yesterday, still picking up tree limbs.
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http://www.sott.net/article/251243-Pilot-photographs-strange-green-glow-in-atmosphere
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Could a planet have a green atmosphere? If you mean, would a human standing on the surface observe the sky as green, then absolutely yes! I have seen it on Earth! Right before a really big thunderstorm or tornado the sky can turn a really weird shade of green. I believe this has to do with scattering effects from dust in the air, but I'm not sure.
Only slight differences can change the apparent color of scattered light: notice how the sky changes all sorts of interesting colors at sunrise and sunset? Each of those could be the "normal" color of the sky if parameters changed a little. Our sky is only blue because that's how it worked out, there's nothing special about the color.
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Could a planet have a green atmosphere and if so, what gases would it consist of?
Fascinating as a discussion or perceived sky colour might be; I suspect it is not what this question is about. If it is about atmospheric colour, I'm with distimpson and chlorine gas. However, the colour attributed to any particular wavelength of light depends on the observer. Any living observer on such a planet is unlikely to be human, and might, therefore, interpret colours differently.
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Good point Bill. Maybe there are animals here seeing it as we would call green? insects, bees etc? What colors would they see?
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The point there, which confuses me thinking of it :) is whether color perception is universally same, or not? Amongst us humans we all seem to agree on green being a specific color. Our eyes work on same basis observing the same frequencies, and I know that I see a specific color, which we all seem to find the same? Can I from that also assume that the color 'green' will present itself the same for all types of eyes/receivers? Is 'green' a 'objective' attribute? Belonging only to one frequency, or is it a combination of what spectrum and what receiver translating that frequency into a color? It's also a question of asking if 'green' can exist as a immaterial object in itself I guess? Or if it is a subjective definition?
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Yor_On,
I think you could ask the question in two ways.
Would all species of animals see the same rainbow? Red, Orange, Yellow, Green, Blue, Purple?
Does a colorblind person perceive the rainbow like a trichrome?
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Funderstandinggraphics.com%2Fwp-content%2Fuploads%2F2009%2F10%2Fcolor-blind-chart.png&hash=2e5aae424557f695f4c0f87fae0e750f)
http://understandinggraphics.com/design/designing-for-color-blindness/
Perhaps a tetrachrome (http://en.wikipedia.org/wiki/Tetrachromacy#Possibility_of_human_tetrachromats) would even see more colors in the rainbow.
This may actually be dependent somewhat on the actual frequency that each cone fires at, which may be somewhat species dependent.
So, all species that live in our atmosphere can perceive a color between say 500 and 550 nanometers, but they might not all perceive it as the same.
A deep ocean species might have much better vision in IR frequencies, and much worse vision in the normal color range that surface dwellers see.
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What can we actually know about the colour vision of others?
Let’s take two of the three colours of traffic lights – red and green. Because Bob always calls the top one red and the bottom one green; and because Alice always does the same; it is easy to assume that they see the colours the same. However, Alice may see green at the top, but because she grew up with the belief that that colour was called “red”, that is what she will call it. How do we know that the colour we all call red is the same for all of us?
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Yes Bill, that's a difficult one. The question seems to become what we mean by communicating a experience, doesn't it? I call it green, you call it green, and we can both agree on any other place that I see as green should be green for you too. So we have a same definition, but is you experiencing 'green' seeing the same as me. That one is the most confusing to me as it asks if there can be a 'objective' experience, or if they are ideas we compromise on, not even knowing that we do so. It's a weird question :)
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What this one goes back to in my mind is a existential question, also belonging to physics. What is a measurement?