This reminds me of a discussion I had with a physicist recently. He insisted that the reason for Mars' atmosphere being gone was its low gravity. While it seemed reasonable to me that this could partly be a factor, I (intuitively) think the lack of a magnetosphere might play a bigger role in atmospheric loss. Did someone here by chance do some calculations that could help settle this argument?

(Note that the argument is NOT about smaller planet sizes leading to earlier magnetosphere failures. It's strictly about what leads to a greater rate of atmospheric loss: Mars' low gravity, or it's defective magnetosphere?)