H2S could stabilize the SO2 ??
And keep it as a "nice" liquid. Even if it does not go to SO3, SO2 combines with water to form sulfurous acid. I have been considering - not in any real scientific way, mind you - that Mars was, and is very reducing. Note the pure iron dust (and the non decomposing olivine). If SO2 was extensive, one needs a mechanism to get rid of it, since the current atmosphere is almost all CO2, a much lighter, less retained gas. Thanks for the info - I was just guessing that Mars was enriched in sulfur. I will read (not eat) that acid paper.