I have developed a fictional planet with the following atmospheric composition;
Is this breathable to humans with an atmospheric pressure similar to Earth's? What about 50%, 200%, and 500% of Earth's atmospheric pressure?
Lastly, what would it look like in terms of color?
insect to breathing. With more O₂ insect could have bigger bodies without suffocate.blue.Albedo of Earth. Less CaCO₃ will reduce the albedo, lower albedo will absorb more of the Sun's heat. How much? I am not god, so I have no idea (maybe despicable).Well, also you won't have some gasses like methane or chlorofluorocarbon that increase the greenhouse effect (but very low or null since your CO₂ increase). Remember that some gasses like ozone are very important for global warming and to stop UV radiation from the Sun, your people will have much skin cancer since O₃ is the only gass that reduce the UV rays <290 nm wavelengh.
Atmospheric Pressure:
If you want to know more about necessary oxygen partial pressure to breath you can check the answers of What is necessary for a breathable artificial atmosphere?
With the excel tables I have calculated this:
$$ \begin{array}{|cc|cc|c|cccc|} \text{Chem}&\text{%}&\text{g/mole}&\text{%}&\text{RMS}&&\text{Pres. (atm)}&&\\ &&&&&1\text{ atm}&0.5\text{ atm}&2\text{ atm}&5\text{ atm}\\ \text{N}_{2}&60.4&14.007&8.460228&729.083468&61.2003&30.60015&122.4006&306.0015\\ \text{O}_{2}&27.6&15.999&4.415724&682.186932&27.9657\text{*}^{1}&13.98285\text{*}^{2}&55.9314\text{*}^{3}&139.8285\text{*}^{4}\\ \text{Xe}&9.8&131.293&12.866714&239.13818&9.92985&4.964925&19.8597&49.64925\\ \text{Ar}&0.64&39.948&0.2556672&431.720126&0.64848&0.32424&1.29696&3.2424\\ \text{CO}_{2}&0.06&44.01&0.026406&411.314605&0.060795&0.0303975&0.12159&0.303975\\ \text{H}_{2}\text{O}&1.5&18.0152833&0.27022925&642.879041&1.519875&0.7599375&3.03975&7.599375&\\ \end{array} $$
I don't know your planet mass. If it's like Earth, no problem. If not, check that your escape speed is less than some gas RMS (remember that I used 25 °C) (or less than a third using the @Alice advice from comment), if escape velocity is greater than the RMS gases from the planet would take millions (maybe even more) of years to escape
10.36 km/s (calculated with 4.8675×10<sup>24</sup> kg of mass and 6,051.8 km of radius). The average mol of Venu's air is 44 g/mol, so his MRS is 645.49 m/s, gas can't escape. For more information you can read this post (in spanish)
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Commented
Jun 6, 2017 at 21:00
27% Oxygen is absolutely breathable. Other components should not affect breathability. At 50% atmospheric pressure, oxygen has an equivalent concentration of 13.8%, which is a little too low for long-term breathing. However, short trips are totally possible. At 200%, equivalent oxygen would be at 55.2%. This would be harmful with long term exposure, but again perfectly fine for a short time. At 500%, we would be getting into the toxic territory. It is similar to the oxygen effects on deep divers.
For the color, the air would still be colorless, however the effect on color of the sky and color of setting sun should be more pronounced in higher pressure.
At 500% you could get anesthetic effects from the nitrogen and possibly the xenon. Xenon is used as an anesthetic but at 1 atmosphere it requires 60-70%. I conclude it must be a more potent anesthetic than nitrogen gas because we already routinely breathe 70% N2 at 1 atm.
Nitrogen narcosis can happen from normal nitrogen-containing air at pressure - I think one can get into trouble between 4 and 5 atmospheres.
Otherwise this looks like normal air with a percentage of oxygen replaced by xenon.
CO2 is a little high.
Breathing does two things: it grabs O2 and it dumps CO2. Of these, the CO2 generally feels more urgent. If you hold your breath for a while and feel your lungs start to burn, you're feeling the effects of excess CO2.
(Hypoxia is also bad for you, but people tend to pass out before they notice it. Pilots get special training in how to spot the symptoms and react to them.)
So you'll have about 50% more CO2 in your atmosphere than Earth. That probably leads to a few rare (and exotic) complications, but also slightly more common acidity in various fluids. Like, tooth decay might be slightly more common on this world. For anyone worried about it though, the easy buffer is calcium cabonate (Tums, but also chalk like for a chalkboard). Natives are likely fine; visitors probably chow down on Tums, and anyone who doesn't drink beer gets kidney stones.
Vistors also likely have their lungs start burning with less exertion than their used to. That probably works about like how exercise in Denver makes you tired more easily, and goes away after a few weeks/months of acclimatization.
Just an addition concerning the colour (as the compositional questions already have been covered):
Rayleigh scattering that turns our atmosphere blue is more efficient the shorter the wavelength. It explains why the sunset is red, because blue is scattered away long before it reaches the observer, and it should make you think why our sky isn't actually purple (which has even shorter wavelength than blue).
The answer to the latter is that our sun emits more blue light than purple, on the steep slope of the black-body function, and there is some purple mixed in in our blue sky, but blue still prevails.
Thus in my understanding your sky colour will change:
At 200-500% pressure, you have essentially the sun-set situation already at noon and during the rest of the day, so the sky will be red-orangeish.
At 50% pressure the blue hasn't been scattered away yet on ground level, so purple might come out better, but you still might want to turn the star a bit hotter so that its purple/blue ratio increases.
This is under the assumptions of pure Rayleigh-scattering, there might be other effects, like high-pressure absorption bands in $N_2$ being activated at high pressure, that could change the colour away from my description.
