[SunkenCiv]

Maybe no spin, but I doubt that a magnetic field has much to do with whether there's any atmosphere, either there, or on Mars, or Earth for that matter.

Venus is known not to have a magnetic field. The reason for its absence is not at all clear, but it may be related to a reduced intensity of convection in the Venusian mantle. Venus only has an induced magnetosphere formed by the Sun's magnetic field carried by the solar wind.

Venus has the most massive atmosphere of the terrestrial planets, which include Mercury, Earth, and Mars.

[Telepathic Intruder]

Venus has substantial atmosphere loss, similar to Mars which once had enough air pressure to support liquid water. But since Venus has so much of it, it will take a long time to deplete. To add to your point, however, the moon Titan has an atmosphere thicker than Earth's, even with the much lower gravity. The difference is the amount of solar wind. Titan only gets about 1% that Earth does.

Earth's magnetic field keeps most of the solar wind from interacting directly with earth's upper atmosphere, which would ionize it and blow it into space similar to what they think happened to Mars.