# I'm trying to create a planet with a purple sky, human life on planet not necessary [duplicate]

This question already has an answer here:

I think that it could be possible with a red sun, and either massive oceans containing algae or a surface covered with active volcanoes. What would my atmosphere be made of, and what color would those oceans/volcanoes appear?

Although humans don't need to exist on this planet, they could theoretically discover it and explore.

Edit: I know that there is a lot more to color than what the atmosphere is made of. I was wondering more what sort of atmosphere would support either the ocean or volcano theory.

## marked as duplicate by Chickens are not cows, Renan science-based StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Feb 19 at 1:17

• I'm looking to get purple, not turquoise, although I did find some of that information useful. – Jess OhYess Feb 18 at 22:16
• Sure Jess OhYess, I hit a dilemma about the question - it's unique. If you look here: en.wikipedia.org/wiki/List_of_colors:_N%E2%80%93Z There are 9 different purples and here: en.wikipedia.org/wiki/Color_vision#In_other_animal_species If you look at just the trichromacy section, there are 10 billion colours we can percieve. We need to draw the line about duplicate questions somewhere. – Chickens are not cows Feb 18 at 23:45
• This would make a good meta question. I'm going to vote to keep open for now but I'd like to hear more about what the community thinks about the duplicate issue. – Cyn Feb 18 at 23:47
• Jess OhYess. Without question I'm no expert on colour, but the point is, if we were to entertain all ten billion possible versions of the question which represent a minute variation in colour sense we'd enter a realm of possible allowable questions that would be phillosophicaly acceptable, but totally impractical to host on the servers. SE.'s model: "While every question deserves a chance to be answered, at some point the annoyance to those searching for a solution outweighs the increasingly-small chance that an answer will be provided." – Chickens are not cows Feb 19 at 1:11
• And @renan... did you know your vote would carry the weight of a moderator's hammer? I want to hear your justification for the duplication, too. We could probably use a meta question with a community wiki answer that wraps all the "what color is my sky?" questions up for easy access by new users, but that lack isn't enough to rationalize this action. – JBH Feb 19 at 15:37

This situation is actually really messy

If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light. Our eyes are also less sensitive to violet. That's part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue. The rest of the answer to this puzzle lies in the way our vision works.

and the answer goes on to explain that issue.

Therefore, a big part of your problem is from whose perspective is the atmosphere purple? Keep in mind that from space you don't see a blue atmosphere. You see blue oceans due to light scattering and reflecting in the water.

It's your story, so ultimately the perspective is that of your readers. Your readers are human. So the atmosphere is purple from the perspective of humans. Except that we really stink at seeing purple in an atmosphere that we can breathe, which means you're only asking this from the perspective of humans in space suits where the faceplate doesn't get in the way of seeing purple light. And whatever is causing the purple must be whomping powerful because we're not designed to see it in the first place (rainbows notwithstanding, go read the entirety of that Physics.SE answer).

The next problem is that atmospheric color has almost nothing to do with star color. We have a really good question on this site about exoplanet sky colors with breathable atmospheres (Terrestrial Exoplanet Skies – I've Built a Visual Sky Chart. Is it Correct?) and what that question demonstrates is that with the exception of clouds and twilight, you'd still see a blue sky even with a red star. This makes sense when you remember that sky color is due to Rayleigh Scattering of the predominant chemicals (in our case, oxygen and nitrogen). This means that you'd need an atmosphere that naturally scattered the purple spectrum and not anything else because we really stink at seeing the purple spectrum.

User Rafael is correct that Iodine would be a good candidate. Remember, it must be the predominant chemical in the atmosphere. If significant amounts of either oxygen or nitrogen exist, we'll see blue because that's how we're designed. And if there's enough iodine in the atmosphere to do that, it would probably eat through the seals of your space suit.

As for what volcanoes and oceans would look like? Well, lava would look red and oceans (assuming they're full of water) would look blue. Atmosphere has nothing to do with what either looks like. Rayleigh Scattering is not a lens that we peer through.

Conclusion

Rafael's comment (which should have been an answer) is correct that Iodine will solve your problem: kinda. No human will ever breathe the atmosphere, and it would only look purple to humans and might not to any other creature — but there you are.

• that is extraordinarily helpful. In my scenario, no human ever WOULD have to breathe on that planet. Your comment about perspective was also enlightening. Is there any way the ocean could appear red? Like if there was enough algae on the surface of the water. So then if my sky was blue... after the refracted light bounced off the 'red' ocean, the sky would then appear purple? Also, this planet does not need to support any flora or fauna. – Jess OhYess Feb 19 at 1:04
• @JessOhYess, there's no way for water to appear red. You could, as you mention, add algae or some other component - or make the fluid something other than water. However, bouncing the light to make the atmosphere purple doesn't work. You're thinking that light can be mixed like paint to get different colors. That's not how light works. The color you see is the predominant refracted (scattered) wavelength. Purple is its own wavelength. Red and blue are actually at the opposite ends of the scale. So, that can't be done. – JBH Feb 19 at 15:30