Well that's stretching a conditional clause a little, IMHO.

Downwind of an active volcanic vent, you might get rain of literally sulphuric acid - more like a scotch mist, given the carrying capacity of the thin atmosphere. This could alter rocks at the surface.

Trouble is, there are no obvious vents close upwind of this site.

Maybe a major eruption of one of the Tharsis centres could inject enough SO2 into the atmoshere to have them rain sulphates out on rocks out here, then attract atmospheric moisture and form a sulphuric acid dew.

Heck, you could do that with sulphate-rich mars dust in the millennia after a major eruption, until it was all consumed by reactions with exposed rocks.

But that isn't "atmospheric water", it's "trace water vapour" - a different animal indeed. And it isn't really "rain", either.

(If it is rain, it's the dardest acid rain you ever saw in your life!)