NASA's twin Mars Exploration Rovers, Spirit and Opportunity, launched toward Mars on June 10 and July 7, 2003, in search of answers about the history of water on Mars and the geology of two distinct landing sites.
The primary science objectives of the MER mission is explore two sites on the Mars that might once have had water, and to assess ancient environmental conditions at those sites and their suitability for life.
In spite of the disappointment early in the mission upon finding that Gusev crater is floored by volcanics, and not lake sediments, roving on the Columbia Hills uncovered a rich assortment of rocks and soils containing evidence for interaction with water. In an area around a feature known as Home Plate, the APXS was key in showing that some light-toned rocks and soils are rich in silica. Similar silica-rich deposits on Earth are commonly formed in areas of hydrothermal activity, for example in Yellowstone, and are habitats for life. On the flanks of the Columbia Hills, the Mössbauer spectrometer was key in showing that the Comanche outcrop included Fe-bearing carbonate minerals much like those in the ancient martian meteorite Allan Hills 84001 recovered in Antarctica. The evidence supports formation of the carbonates in hydrothermal solutions at near-neutral pH, again conditions that would be benign for life.
Opportunity has shown that the Burns formation sandstones include sediments deposited in shallow water, and that ground waters were responsible for post-deposition diagenesis. This process resulted in formation of abundant, mm-sized hematite concretions. These concretions are more resistant to physical and chemical weathering, and their concentration on the surface of Meridiani Planum as a lag deposit is the cause of the strong hematite signature detected from orbit. The Mössbauer and APXS instruments were important players in identifying hematite as the mineral composing the concretions. Other styles of aqueous processes were uncovered on the Endeavour crater rim. Light-toned veins from mm to cm widths are often found, and data returned by the APXS has shown that these veins are composed predominantly of calcium-sulfate. Calcium-sulfate veins are common features in rocks on Earth where they are formed by precipitation from aqueous fluids flowing through the rocks. One exceptional vein system was composed not of calcium-sulfate, but of material rich in silica and alumina as demonstrated by the APXS. The unusual composition of this veining material points to a distinct aqueous environment at this location.
Read more about ARES scientists' research based on their MER mission work:
Gellert, R. et al., including D. W. Ming (2004) Chemistry of rocks and soils in Gusev Crater from the Alpha Particle X-ray Spectrometer. Science 305, 829-832.
Morris, R.V. et al. (2004) Mineralogy at Gusev Crater from the Mössbauer spectrometer on the Spirit Rover. Science 305, 833-836.
Squyres, S. et al., including R. V. Morris, and D. W. Ming (2008) Detection of silica-rich deposits on Mars. Science 320, 1063-1067.
Squyres, S.W., including D. W. Ming, D. W. Mittlefehldt and R. V. Morris (2009) Exploration of Victoria Crater by the Mars Rover Opportunity. Science 324, 1058-1061.
Morris, R.V. et al., including D. W. Ming (2010) Identification of carbonate-rich outcrops on Mars by the Spirit rover. Science 329, 421-424.
Squyres, S.W. et al., including D. W. Ming and D. W. Mittlefehldt (2012) Ancient Impact and Aqueous Processes at Endeavour Crater, Mars. Science 336, 570-576.
Arvidson, R.E. et al., including D. W. Ming, D. W. Mittlefehldt and R. V. Morris (2014) Ancient Aqueous Environments at Endeavour Crater, Mars. Science 343.