[nomel]

> air, water

If all the water on mars were melted, it would cover the planet in 100ft of water [1]. With the safe assumption of any kind of water recycling, and indoor habitats, there's plenty of water on mars, which also means there's plenty of oxygen on mars (along with the 95% CO2). The atmosphere is 3% nitrogen, and 1.6% argon, which means there's plenty of air on mars. You don't have to fill the sky with air, just the buildings.

[1] https://marsed.asu.edu/mep/water#:~:text=Taking%20what%20can....)