Thats lEAking atmosphere and that kind of proccess isn't a fast one (unless you try to give an asteroid an atmosphere), it happens over millions, even billions of years. Also, Mars has a thinner atmosphere because it lost it's magnetic feild (which keeps the solar wind from stripping away the atmosphere), so I don't know how much thinner it would have been with a magnetic field. Aside from the presence of a magnetic field (Saturns in the case of Titan), there are lots of factors in a dense atmosphere. Venus is slightly smaller than Earth and it has an atmospheric density akin to the deepest parts of the ocean.
Still though, the idea of having measurable atmospheric density is a nice one. Since there is going to be atmospheric drag, maybe there will be a measurable density?
I wonder how much more complex the atmospheric model will be in the major release....