The problem for Mars is that it already has as much atmosphere as it can hold in the presence of solar wind and solar UV radiation, both of which strip away atmosphere, taking the lightest components first.I don't know if anyone already said this, but we could restore Mars's atmosphere by making a few factories and releasing enough CO2 to keep heat in. Then we can terraform Mars.
Yes, there are substantial carbonate deposits on Mars, though not enough to provide significant atmosphere. But release it and it will be lost to space, probably as rapidly as you can produce it.
It isn't just a problem for Mars either. The dinosaurs had a thicker atmosphere than we do today. Twice as early, the coal forests grew in an atmosphere that is estimated to have been thirty percent thicker than ours. Earth is also losing atmosphere, slowly.
These are the reasons that we presently believe that a Martian Civilization will be essentially an indoor civilization.
But then, so is New York, or any other quite large city essentially.
That's the bad news. The good Anews is tat it can be done, using technology we have today, or even what we had thirty or forty years ago. Life would be hard at first, but after a while it could be quite comfortable. Food, clothing, air, water and food can be produced and in abundance if given time and effort. The people who are wiling to live with the problems in order to eventually have the benefits are called Pioneers. Pioneers are the people who built the USA.
However, Mars won't be the end product of civilization. On this thread, there have been some advocating building totally artificial habitats in space. Jeff Bezos is on record as being in this camp.
If we tried Mars now, as Robert Zubrin and Elon Musk want us to, there are a great many things we don't know that might just kill us (or them. I'm probably too old to be considered.)
But going to the Moon first, along with building some large orbital facilities to prepare the way and in the meantime, starting large industry off planet would not only pay for all the effort we as a species have put into space flight, it will also point out to us as a species what is important there.
Some folks want us to just send colonies directly to the asteroids. Such folks commonly refer to planets as 'Gravity Holes". Instead of holes they sometimes say wells or traps.
Personally, I think we will do all three. First off should be establishing economically self sufficient bases on the Moon. It's just closer. This moon group will import the things it needs that it can't make itself from Earth. That amount imported needs to be as small as possible because it's very expensive to import things from Earth. Recycling will be draconian even when compared to Greenpeace proposals. When it costs thousands of dollars per kilogram for food or anything else, you don't waste anything. What's true for food is also true for clothing and tools.
What is important for the Moon is vital for Mars. On the Moon you are a week away from emergency resupply. On Mars, it's up to three years away. That's a long time to have to hold your breath.
When we can comfortably live on the Moon, then we can successfully tackle Mars. Then we will know what the issues actually are. As it is, we are only guessing. Not knowing what we don't know or not even suspecting where the holes are in our understanding of what is needed is not a place I want to be.
So yes, colonize Mars, but first colonize tbuilding bases and towns in HEO (High Earth Orbit) along with it will make the asteroid miners of SF an achievable reality along with it.
It's not pick one of the three options as the only thing we can do, it's doing all three as we are able.