Abundant sulfates appear to exist on the surface of Mars and have commonly been attributed to a planet-wide volcanogenic ‘acid fog’—where clouds of acid mist react directly with surface rocks or acidify surface waters—from which the sulfates precipitated [e.g., Clark and Baird, 1979]. In particular, Meridiani Planum, a plain located just south of the Martian equator, hosts many sulfate minerals.
Squyres et al. [2004, 2006] and Squyres and Knoll  hypothesized that the sulfate minerals there, particularly acid sulfates such as jarosite, precipitated from high concentrations of sulfuric acid in flowing and standing water and groundwater. However, such explanations are problematic, because magnesia and alkali-bearing minerals in the basaltic regolith should have been able to neutralize sulfuric acid rapidly, making precipitation of acid sulfates such as jarosite impossible.