I've been reading this wikipedia article on terraforming Mars. One of the major issues with terraforming Mars is essential molecules and elements escaping the planet. There have been several proposed solutions to stopping solar wind degradation that seem feasible, however it still leaves the possibility of molecules escape velocity being low enough to leave the planet easily, especially if the planet were to be heated up.
According to this graph, it looks like water vapor would easily escape Mars's atmosphere:
What could we conceivably do to keep liquid water exposed on the surface of Mars when any evaporation could leak large amounts of water from the planet?