[simulate]

Yes, it is possible that with a magnetic field the moon's gravity would be sufficient to retain an atmosphere. Titan has an atmosphere denser than earth's atmosphere despite having a lower gravity than the moon. There are a few hypotheses why and one of them is that, although Titan has no magnetic field of its own, it orbits inside Saturn's magnetosphere. https://astronomy.stackexchange.com/questions/8345/how-does-...