Hidden opportunity?
Hi Nick
Could there be enough CO2 under the surface that it could be liberated with a few well placed nukes or impactors? Sub-terranean lakes of liquid CO2 would make terraforming the place a pretty quick task - at least if all you wanted to do was thicken the atmosphere to allow people in oxygen masks access to the surface - tho the peroxide dust might be a challenge for a few years until rain could be engineered.
A cold, dead Mars is a new terran-derived biosphere in embryo, awaiting some bio-friendly engineering.
Adam