Re:Magnetic field. (Score 4, Insightful)

It's not clear whether Mars had a thick atmosphere to lose. If it did, though, there's only two things that could have happened to the heavy molecules (e.g. CO2) that it contained: they could have been sequestered in reservoirs such as carbonates (not much of which have been yet found), or they could have been lost to space through non-thermal processes. In the last 3.5 billion years on so, the major non-thermal loss process is sputtering by the solar wind, as you say.

Whether a terraformed planet is usably terraformed is a matter of time scales. Models suggests that between 0.1 and 3 bar CO2 could have been lost through sputtering over a period of about 3.5 billion years. Taking the maximum rate, this is an annual loss of less than 1 part per billion per year, or 0.1 bar in 100 million years. Thus no significant loss due to sputtering would occur on the time scale of human civilization.

I vaguely recall seeing calculations for the duration of an atmosphere on a terraformed Moon. IIRC, even such an atmosphere might last for useful time scales; a 1 bar Earth-like atmosphere might last for several thousand years before being lost due to thermal escape and sputtering.