re: Is the lighter colored rock dissolving?
No need to dissolve (which is unlikely into a thin gas, anyway).
More likely, it is being weathered away by sandstorms - it is much softer than the marbles, which armour the surface of the plains, and stick out dramatically from the eroding rock.
of course, it *could* be dgrading into other compounds plus SO2 gas (e.g. when irradiated), but we see so little SO2 on the atmosphere that it must be a very slow process.
Me, I prefer sandstorms - the local "soil" consists of dark and pale grains - relics, respectively, of the marbles and the rocks?