@Setanta,
The relative gravity of Mars is 38% of one G (the force of "gravity" on our planet at sea level). My plan to use snowballs from Saturn would have the one drawback that you'd need to keep cartin' the **** in. It's not the gravity well which is responsible for the thin atmosphere on Mars, it's the lack of a magnetic field. The solar wind is a serious bitch if you don't have some really mega-sun block . . .
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In term of human life times or even 10s of thousands of years off hand I do not think that any large part of the atmosphere would leak away solar wind or no solar wind or a lower g field then earth.
Hit Mars with some big water full comets and I do not think you would need to repeat it for perhaps a few 100s thousands years even.
If some one could loan me access to a super computer for a few months I might be able to give you a better idea then that however.