Mars can retain water vapor just fine. It occasionally snows lightly. Without a magnetic field (which could be built easier than terraforming), over 100 millio

Mars can retain water vapor just fine. It occasionally snows lightly. Without a magnetic field (which could be built easier than terraforming), over 100 million years, it can slowly lose atmosphere, but nothing like a relevant rate for humans. The sun is also getting significantly hotter on a similar timescale, which makes habitation on Venus (And Earth, for that matter) significantly harder and dependent on active geoengineering methods.