Contrasting styles of water–rock interaction at the Mars Exploration Rover landing sites

The nature of ancient hydrological systems on Mars has been the subject of ongoing controversy, driven largely by a disconnect between observational evidence for flowing water on the Martian surface at multiple scales and the incompatibility of such observations with theoretical models that predict a cold early Martian environment in which liquid water is unstable. Here we present geochemical data from the Mars Exploration Rovers to evaluate the hydrological conditions under which weathering rinds, soils, and sedimentary rocks were formed. Our analysis indicates that the chemistry of rinds and soils document a water-limited hydrologic environment where small quantities of S-bearing fluids enter the system, interact with and chemically alter rock and soil, and precipitate secondary mineral phases at the site of alteration with little to no physical separation of primary and secondary mineral phases. In contrast, results show that the sedimentary rocks of the Burns Formation at Meridiani Planum have a chemical composition well-described as a mixture between siliciclastic sediment and sulfate-bearing salts derived from the evaporation of groundwater. We hypothesize that the former may be derived from the recently investigated Shoemaker Formation, a sequence of impact breccias that underlie the Burns Formation. This result has important implications for the style of chemical weathering and hydrology recorded by these sedimentary materials, revealing long-range transport of ions in solution in an open hydrological system that is consistent only with subsurface or overland flow of liquid water.