[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.

[RightWhale]

Since it’s gone we can only speculate. We can’t even speculate, since you need some evidence before speculation.

[UCANSEE2]

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

Maybe Mars came out of a LITTLE ICE AGE of it’s own, for the same reason the Earth, and Venus, and some other planets have.