We can get methane and oxygen from the martian atmosphere. https://en.wikipedia.org/wiki/Sabatier_reaction#Manufacturing_propellant_on_Mars https://en.wikipedi

We can get methane and oxygen from the martian atmosphere. https://en.wikipedia.org/wiki/Sabatier_reaction#Manufacturing_propellant_on_Mars https://en.wikipedia.org/wiki/Sabatier_reaction#Manufacturin... This can be elaborated to produce plastic feedstocks. However, for making an earthlike atmosphere, we'd need to import nitrogen. For more, check out The Case for Mars by Robert Zubrin.