The Martian rover Opportunity has traversed the valley called Meridiani Planum, and convinced scientists that the region was once bathed in an acid lagoon. Now scientists believe the region was fed by non-acidic groundwaters destroyed by the drying planet.
A group of Earth scientists and chemists published a paper over the weekend in Nature Geoscience that offers an intriguing theory about what may have happened to the waters of Mars. The researchers suggest that Meridiani Planum was flooded with groundwaters that were pH neutral (like fresh water on Earth) but high in iron. As this water bubbled to the surface, the young Martian atmosphere quickly oxidized the iron, leaving behind a highly acidic water (like the reddish water you can see on Earth in the so-called "blood falls" of Antarctica).
According to a release about the study:
Joel Hurowitz and colleagues used data obtained by Opportunity to assess the geochemical pathways that could have led to the formation of the rocks. Their geochemical calculations showed that as iron-rich, fairly neutral groundwater reached the surface, the iron could have been rapidly oxidized by exposure to ultraviolet radiation or atmospheric oxygen. The resulting chemical reactions would have acidified the water remaining on the surface.
This is also further confirmation that a lot of water could lie frozen beneath the Martian poles. It may be full of iron, but at least it won't be made of acid.