The Atmosphere of Mars is not breathable. The pressure is too low, and there is too little oxygen. And yet, it gives Mars something that makes it the most habitable of all planets in our solar system, except Earth of course. It provides valuable chemicals, and it forms a visible sky, mostly from dispersed dust.
Composition (gaseous parts)
|96.0%||Carbon dioxide (CO2)|
|0.09%||Carbon monoxide (CO)|
|0.03%||Water vapor (H2O)|
|trace||Neon (Ne), Krypton (Kr), Xenon (Xe), Ozone (O3), Methane (CH4)|
Mars surface pressure
1-9 millibars (depending on altitude) - 600 Pa, average.