Space Science Reviews

, Volume 174, Issue 1, pp 329-364

First online:

Geochemical Consequences of Widespread Clay Mineral Formation in Mars’ Ancient Crust

  • Bethany L. EhlmannAffiliated withDivision of Geological and Planetary Sciences, California Institute of TechnologyJet Propulsion Laboratory, California Institute of Technology Email author 
  • , Gilles BergerAffiliated withIRAP, CNRS-Université Toulouse
  • , Nicolas MangoldAffiliated withLaboratoire Planétologie et Géodynamique de Nantes, CNRS/Université de Nantes
  • , Joseph R. MichalskiAffiliated withPlanetary Science InstituteMineralogy, Natural History Museum
  • , David C. CatlingAffiliated withDepartment of Earth and Space Sciences/Astrobiology Program, University of Washington
  • , Steven W. RuffAffiliated withSchool of Earth and Space Exploration, Arizona State University
  • , Eric ChassefièreAffiliated withLaboratoire IDES, UMR 8148, Université Paris-Sud, CNRS
  • , Paul B. NilesAffiliated withAstromaterials Research and Exploration Science, NASA Johnson Space Center
  • , Vincent ChevrierAffiliated withW.M. Keck Laboratory for Space and Planetary Simulation, Arkansas Center for Space and Planetary Science, University of Arkansas
    • , Francois PouletAffiliated withInstitut d’Astrophysique Spatiale, Université Paris-Sud

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Clays form on Earth by near-surface weathering, precipitation in water bodies within basins, hydrothermal alteration (volcanic- or impact-induced), diagenesis, metamorphism, and magmatic precipitation. Diverse clay minerals have been detected from orbital investigation of terrains on Mars and are globally distributed, indicating geographically widespread aqueous alteration. Clay assemblages within deep stratigraphic units in the Martian crust include Fe/Mg smectites, chlorites and higher temperature hydrated silicates. Sedimentary clay mineral assemblages include Fe/Mg smectites, kaolinite, and sulfate, carbonate, and chloride salts. Stratigraphic sequences with multiple clay-bearing units have an upper unit with Al-clays and a lower unit with Fe/Mg-clays. The typical restriction of clay minerals to the oldest, Noachian terrains indicates a distinctive set of processes involving water-rock interaction that was prevalent early in Mars history and may have profoundly influenced the evolution of Martian geochemical systems. Current analyses of orbital data have led to the proposition of multiple clay-formation mechanisms, varying in space and time in their relative importance. These include near-surface weathering, formation in ice-dominated near-surface groundwaters, and formation by subsurface hydrothermal fluids. Near-surface, open system formation of clays would lead to fractionation of Mars’ crustal reservoir into an altered crustal reservoir and a sedimentary reservoir, potentially involving changes in the composition of Mars’ atmosphere. In contrast, formation of clays in the subsurface by either aqueous alteration or magmatic cooling would result in comparatively little geochemical fractionation or interaction of Mars’ atmospheric, crustal, and magmatic reservoirs, with the exception of long-term sequestration of water. Formation of clays within ice would have geochemical consequences intermediate between these endmembers. We outline the future analyses of orbital data, in situ measurements acquired within clay-bearing terrains, and analyses of Mars samples that are needed to more fully elucidate the mechanisms of martian clay formation and to determine the consequences for the geochemical evolution of the planet.


Mars Clay minerals Phyllosilicates Weathering Alteration Geochemistry Mineralogy Noachian