[RightWhale]

They tell us that the martian atmosphere is all CO2

Mars lost most of its atmosphere. It could easily hold a much denser atmosphere than it has. Was it all CO2? Not likely.

[Perchant]

Mars lost most of its atmosphere. It could easily hold a much denser atmosphere than it has. Was it all CO2? Not likely.

That's a red planet herring. If that denser atmosphere was due to SO2, Mars should have been colder.