Oh I'm not saying put an atmosphere on mars, i'm saying using whats already there as a resource for anyone who wants to visit. Chill in a your little landing pod, scoop up some martian dirt and make something you can breathe out of it. While you're at it, use that iron for something useful too, like buildings. I don't think H2O2 naturally occurs on mars... but that'd be cool if it did. Actually, the only thing I really know that exists on mars is iron oxide, is it Fe2O3 or Fe3O4? Regardless, whatever you need to do to free the oxygen atom is the same, right?