For some reason I always think about that in the context of hypotetical self-sustaining Mars colonies. In internet discussion there is always talk about the bi

For some reason I always think about that in the context of hypotetical self-sustaining Mars colonies. In internet discussion there is always talk about the big resources: We'll get oxygen out of water ice, methane out of the athmosphere, we'll get iron out the the regolith, we will farm in greenhouses, etc. But a sustaining colony need rubber gaskets for airlocks and a million little things which in end effect need the whole of Earth's supply chain behind it. Can you make an economic case for Mars if you need to transport ca. 99% of the needed manufactured goods to the red planet first and do that for centuries? What do you sell in return? What is the equivalent to Marsian beaver furs?