[Grig]

Forget about terraforming Mars into an earth-like place anytime soon. Mars, although similar in size to earth, is actually much lighter than earth and doesn't have the gravity to hold onto an earth like atmosphere. If Mars suddly had an earth-like atmosphere right now, the oxygen would start bleeding off into space and only heavier gasses like CO2 would be left behind.

[Lucius Cornelius Sulla]

the oxygen would start bleeding off into space

My memory of this is that it would take millions of years for this process to advance very far.