[ajross]

> what's gravity got to do with it?

When the speed of molecules in the atmosphere (thermal, and thus dependent on molecular weight) approaches escape velocity for the body, the atmosphere starts to leak that component. Gasses like N2 (molecular weight 28) and oxygen (32) will "stick" to Earth where Helium (4) won't. Mars can hold CO2 (44) but not O2.

[adrianN]

But it takes a billion years or so for the atmosphere to erode away. That's plenty of time to get some ROI out of terraforming Mars.