Could the sky be a different color but the atmosphere still be breathable by humans?

Question

I'm trying to explore possibilities for differences between earth and a different planet in my story. I'm wondering if I could change the color of the sky. When doing research on the subject I got a few mixed answers, some saying no, some saying yes. I'd like to know variables that wouldn't negatively effect breathing but still change the color of the sky. If there is anything that could change it, what would other effects of it be.

Answer 1

Yeah, why not?

The reason the sky is blue is because of nitrogen content. Raleigh scattering also accounts for some of it, but the sky isn’t purple and other planets with atmospheres have colors depending more on their atmospheric composition than the density (as such scattering would imply). For Earth, most of the atmosphere is nitrogen, which tends to absorb blue less than other colors. As such, more blue light gets to our eyes.

Humans (and most fauna as we know it) don’t even care about nitrogen. We breathe it in and then just back out, using instead the oxygen. So, what if you replaced some of the nitrogen with, say, a noble gas? Inert and safe enough to breathe so long as oxygen isn’t displaced, these gases have different more interesting colors (especially when placed in the upper atmosphere, where they may become ionized and start emitting light as well as absorbing it). Increased helium content in the upper atmosphere might make the sky red, and if you could replace some of the nitrogen with krypton, you could turn the sky purple.

But will I suffocate?

As long as you don’t actually decrease the relative amount of oxygen in the air, it should be fine. Keep in mind the properties of air like the speed of sound and how deep your voice is might change due to the change it atmospheric composition of you do this.

Would this be an effective use of the world’s (or other worlds’) supply of noble gases?

Of course not, it would take many, many years of terraforming unless something magical happens, but it would make the sky look cool!