Besides low gravity and high doses of radiation, one of the more intractable difficulties in terraforming Mars is that if we build its atmosphere back up, then presumably it would lose it again via the same mechanisms it lost it the first time (ie solar wind, warmth, and low gravity). However, this may or may not occur quickly enough to really matter to prospective terraformers. Given that it takes considerable atmospheric pressure to allow for the existence of liquid water, and as the general view seems to be that Mars had liquid water during its Noachian and Hesperian eons, it seems logical to suppose that it takes millions of years for Mar's atmosphere to become mostly lost. So imports of gases that mimic the rate outgassed by volcanoes during those eons are sufficient to keep Mar's atmosphere dense indefinitely. Am I missing something here?