The rovers Spirit and Opportunity touched down on Mars in January 2004 and have been conducting extensive observations with the Athena science payload. Together the two rovers have traversed more than 50 km.
Spirit, located on the floor of Gusev crater, investigated basaltic plains, as well as older materials in the Columbia Hills. The rocks of the Columbia Hills are granular and have undergone significant alteration by water. They appear to be largely a mixture of altered impact ejecta and explosive volcanic materials. Spirit discovered silica-rich deposits that may have formed in a hot spring or volcanic fumarole environment, as well as massive carbonate-rich rocks.
Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks are sandstones formed by wind and water erosion and re-deposition of “dirty evaporite” materials rich in sulfate salts. While liquid water was present below and occasionally at the surface, the ancient environmental conditions recorded are dominantly arid, acidic, and oxidizing, and would have posed significant challenges to life.
Opportunity has also investigated the rim of Endeavour Crater, which predates the Meridiani sediments. Basaltic breccias produced by the impact form the rim deposits, with stratigraphy similar to that observed at similar-sized craters on Earth. Localized Zn enrichments suggest hydrothermal alteration of rim deposits. Gypsum veins precipitated from low-temperature aqueous fluids cut sedimentary rocks adjacent to the crater rim. The most ancient rocks of the crater rim have undergone substantial alteration by circum-neutral pH fluids, creating localized zones along fractures that are rich in Al-phyllosilicates. The environment these materials represent would have been more favorable for life than seen elsewhere at Meridiani.