The acidic surface waters that once bathed Meridiani Planum on Mars could have been generated by interactions between groundwater and the early martian atmosphere, according to a paper published online this week in Nature Geoscience. Meridiani Planum, visited by the NASA rover Opportunity, is host to a number of rock formations thought to be formed in acidic water.

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.

The team suggests that the prevalence of the acidic surface water was a consequence of the martian climate drying.