The Martian atmosphere
Mars's atmosphere is so thin that it is more than a hundred times lighter than that on Earth. Atmospheric pressure at ground level is a fraction of ours, and about the same as it is 35 kilometres above the Earth. This thin atmosphere consists almost entirely of carbon dioxide with small fractions of nitrogen, argon, oxygen, and a few other gases. When the carbon dioxide condenses as ice at the poles every winter, the atmospheric pressure plummets by as much as 30%.
Mars's atmosphere is also highly oxidising. Something in the air turns iron in the ground rusty, giving Mars its red colour, but no one knows precisely what. Ultraviolet light, which hits the ground because Mars has no ozone layer to stop it, is almost certainly one cause, but there could be others.
Mars's atmosphere may be cold, dry, thin and inhospitable today - but that may not have always been the case. If liquid water did flow and collect on the surface, as the evidence suggests, atmospheric pressure must have once been higher than it is today and temperatures warmer. The atmosphere would also have been a lot denser and richer with greenhouse gases. Scientists think that much of these greenhouse gases have simply floated off into space.