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How could a planet which humans could land and survive on with no space suits or other help have a green atmosphere? The only requirement is that humans landing here would have to be able to breathe the atmosphere, and potentially live on the planet for their entire natural lives.

Note - I would like to know the percentage of different gases for your answer. For example, if you replace 20% of the atmosphere (in H) and replace it with imagineum then your atmosphere would appear green.

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    $\begingroup$ Still wondering what do you mean with "have a green atmosphere"; should it be the way the sky looks from ground (like ours here is blue usually), or do you mean that thin hull of gas you might see from space if you are behind the planet and watch the sun glare at its edges (... both would have the same color, wouldn't they?)? Or are you after green clouds and fog? $\endgroup$ – Confused Merlin Feb 17 '16 at 6:30
  • $\begingroup$ @ConfusedMerlin your name fits the comment very well $\endgroup$ – vanillagod Feb 17 '16 at 9:48
  • $\begingroup$ @ConfusedMerlin I am talking about on the surface. Kind of like how ours is blue. $\endgroup$ – Xandar The Zenon Feb 17 '16 at 13:14
  • $\begingroup$ @XandarTheZenon Now I'm confused. If by on the surface, do you mean how the sky is seen so a green sky instead our blue? If it's the surface that's seen as blue, then in the case of the Earth it's the oceans that are seen as blue. In that case, it's question of oceans being green instead of blue. Now I think I'm more confused than ever. Although I like the idea of Confused Merlin's green clouds and fog. $\endgroup$ – a4android Jul 7 '16 at 9:21
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I would guess it would be easier to change the sun the planet is revolving around. Blue light scatters the most and the colors go down from there. So if you have say a red dwarf, that is producing a lot less blue, then green is the next 'most scattered' color and with blue mostly missing, green should be prominent.

Other other way would be to have something in the atmosphere that absorbs blue light.

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There are two possible ways I know of why a planet's sky could be green.

The planet has a super-abundant biosphere and the atmosphere is full of, what can be only called, aerophytoplankton. Tiny plant life that float in air the same way marine phytoplankton floats in the sea.

The planet's crust has a high copper mineral content and the atmosphere is full of the dust of greenish copper minerals and compounds. This like the situation where the sky of the planet Mars looks reddish due to the colour of dust floating in its atmosphere.

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There are humans which have colour blindness which makes them see colours differently.

So theoretically, if a human has severe enough colour blindness, they may perceive our Earth skies as greenish.

Humans can survive on this new world, which implies that other life could already exist there.

If one of these alien organisms carried a virus (or something similar) which could infect the humans with non-lethal colour-blindness, then these humans would perceive the sky on this alien world as green. The virus could even be airborne, so they would catch it straight away after breathing the air and immediately start seeing green skies instead of blue.

This would also mean the new planet could have similar atmospheric composition to our own, which would satisfy the condition that humans need to survive there unaided, while also having the atmosphere appear green.

I know this isn't changing the atmosphere to be green, but colour is a matter of perception anyway ;)

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A layer of Chlorine gas in the atmosphere would make it green, the problem with this is that it would also make it very dangerous for humans so you would need to separate the Chlorine and somehow keep it away from your observers.

This could be done either by having isolated pockets Chlorine-free or by somehow restricting the Chlorine to a specific band in the atmosphere similar to our Ozone layer.

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  • $\begingroup$ Nice idea but Chlorine is more dense than Oxygen, so I think that it's more likely the Chlorine would be near the surface and the oxygen would be floating inaccessibly in the sky (if they didn't just mix) $\endgroup$ – colmde Jun 8 '18 at 8:05
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    $\begingroup$ @colmde Ozone is twice as dense as Nitrogen and yet we have an ozone layer. Atmospheric dynamics is complicated :) $\endgroup$ – Tim B Jun 8 '18 at 8:37
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Even on Earth, there are times when the sky can appear green - generally before a storm. The exact reason for this is debatable, but it is probably related to the redness of sunset (itself caused by the sunlight passing through more atmosphere) combining with increased water droplets in the air.

On a planet where such conditions were normal, the sky would always appear green to a human visitor. However, it would also be dimmer overall and all blues would be muted, since less blue light is getting through the atmosphere. And to a species adapted for seeing under these conditions, the sky would probably appear to be at the short-wavelength end of their visible spectrum.

Transparent gases, as a rule, scatter more short-wavelength light than long-wavelength light, so unless you're blocking out light with colored gases, dust, or living organisms (all of which will make the surface appear dimmer) the sky will always appear blue.

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