Chemical and Mineralogical Constraints for Modeling Hydrous Environments on Mars from the Mars Exploration Rovers

Introduction: The twin Mars Exploration Rovers (MER) Spirit and Opportunity have explored the Martian surface at Gusev Crater (GC) and Meridiani Planum (MP) for more that 1840 sols (more than 5 Earth years). Their science payload [1] includes instruments for mineralogical (Mössbauer (MB), Mini-TES, and Pancam) and chemical (APXS) analysis and a tool (RAT) for brushing and grinding rock surfaces. Rover wheels are used to expose subsurface soil. We will focus here primarily on results from the MB (Fe mineralogy) and APXS instruments and the constraints they provide for modeling hydrous environments. Fe-bearing phases: The concentration and minera-logical speciation of Fe constrain the nature and progress of alteration processes by the extent of its mobility in hydrous environments, by its distribution between oxidation states (Fe 2+ and Fe 3+), and by its min-eralogical speciation. Fourteen Fe-bearing phases have been identified by MB [2-4]: 5 primary igneous silicate/oxide phases (olivine, pyroxene, ilmenite, magnetite, and chromite (one occurrence)), 7 secondary phases (nanophase ferric oxide (npOx), hematite, goethite, jarosite, py-rite/marcasite (one occurrence), ferric sulfate, and an unidentified Fe 3+ phase referred to as Fe3D3), and 2 meteorite phases (metallic-Fe and troilite). NpOx is ubiquitous and associated in various proportions with basaltic soils and many basaltic rocks, and it is interpreted to be the primary Fe-bearing component in martian dust. Goethite, along with hematite, is found in highly altered rocks in the GC Columbia Hills at West Spur and Husband Hill. Jarosite, hematite and Fe3D3 are components of the S-rich outcrop at MP. In addition , hematite-rich spherules (a.k.a. blueberries) are imbedded in the outcrop and are present as a lag on the surface. Ferric sulfate is found subsurface in GC on Husband Hill and at several locations near Home Plate. The occurrence of the sulfide pyrite/marcasite is a float rock on Home Plate. Goethite and jarosite both contain OH in their structure and therefore formed in the presence of H 2 O. Ferric sulfate also reasonably requires H 2 O for its formation. Constraints on chemistry, mineralogy, formation pathways: We next combine the Fe-mineralogy discussed above and APXS chemical data [5-8] that together provide ground-truth for modeling hydrous environments on Mars. We will not consider numerical