Take a deep breath. What do you think you just breathed in? Mostly Nitrogen, about a fifth of that breath was Oxygen and the rest was a mix of other gases. To get the same amount of oxygen from one Earth breath, you’d have to take around 14,500 breaths on Mars! With the atmosphere being 100 times less dense, and being mostly carbon dioxide, there’s not a whole lot of oxygen to breathe in.
(Source: NASA)


“Red planet”