# Acid fog on Mars?

Recently a study on the data from the Spirit rover in Gusev crater suggested that there may be acid fog changing the rocks.

What exactly is this acid fog?

How is it formed on Mars and where?

Is it a potential hazard for astronauts and Mars missions?

One obvious solution is $\mathrm{CO_2 + H_2O \rightarrow H_2CO_3}$ which is a very weak acid, but an acid nevertheless, though I doubt it could contribute much to erosion.
The detected perchlorates in soil though are a different matter. There's a range of reactions of $\mathrm{KClO_4}$ and $\mathrm{NaClO_4}$ with water and input of electrons (from cosmic radiation) that result in a range of chlorine-based acids: $\mathrm{HCl}$, $\mathrm{HClO}$, $\mathrm{HClO_2}$, $\mathrm{HClO_3}$ - and these are acids that can cause serious damage.