Science Priorities for Mars Sample Return
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V.E.RegolithThe Martian regolith reflects interactions between the crust and the atmosphere, the nature of rock fragments, dust and sand particles that have been moved over the surface, H2O and CO2 migration between ice and the atmosphere, and processes involving fluids and sublimation. Understanding regolith chemistry and mineralogy is vital to determining the fates of any organic constituents. Some aspects of regolith studies necessarily overlap studies of the local rock petrology, geochemistry, and hydrothermal and low- temperature alteration processes. Although global-scale transport processes may have homogenized much of the fine-grained Martian regolith components, as shown by the similarity of most Viking and Pathfinder soil compositions (e.g. Carr, 2006), the MER rovers have demonstrated that the regolith also contains a diverse range of mineral assemblages, some of which originated locally. Other materials, such as volcanic ash (Wilson and Head, 2007) and impact glass (Mustard and Schultz, 2004), may have come from greater distances. Understanding the mechanisms by which these assemblages are produced is necessary in order to understand the evolution of the Martian surface and key fluid processes. The recent identification of a silica-rich component in a Gusev crater soil deposit that perhaps formed though hydrothermal processes (Ruff et al. 2007) and the presence of hematite spherules in the Opportunity soil (Squyres et al. 2004) highlight the importance of regolith studies. The mm-scale alteration rinds identified on rocks in the regolith in Gusev might have resulted from the reaction of S- and Cl-bearing species with minute amounts of liquid water (Haskins et al., 2005). Studying the mineralogy of alteration rinds within regolith granules would give an insight to water and oxidation processes on Mars over long timescales (MacPherson et al. 2001). A returned regolith sample would likely be evaluated in the following way: Size distribution studies of regolith particles may yield information about local vs. distal provenance, as they did for Apollo regolith samples (McKay et al., 1974). Studies of regolith minerals and their morphology (by SEM, TEM, FTIR, and raman spectroscopy techniques) and the chemistry of various lithologies within the regolith (SEM, TEM and EMPA) can help to quantify the mobility of water, weathering processes, diagenesis, and chemical alteration in Martian regolith, as has been done for Martian meteorites (Gooding et al., 1988; Velbel, 1988; Treiman et al., 1993) and Antarctic dry valley soils (Gibson et al., 1983; Wentworth et al., 2005). Through studies of major elements, water soluble cations (Na+, K+, Ca2+) and anions (Cl-, SO42-, NO3-),the relative extent and importance of the aeolian, salt-rich, seasonally active, and permanently frozen soil horizons can be determined, and should be possible to evaluate for martian regolith as well. Finally, we already know that Martian impact glasses contain trapped atmospheric gases (Bogard and Johnson, 1983), and the regolith could be an ideal sample in which to find this component. Gas release studies would be important to interpret the history and evolution of the Martian atmosphere. Finally, a regolith sample would be used for toxicity tests, including intratracheal, corneal, dermal and ingestion studies. The mixed and complex nature of regolith samples could lead to unexpected findings. For example, Bandfield et al. (2003) proposed that atmospheric dust on Mars contains a few percent carbonate. This is important because carbonate provides a record of atmosphere-water-crust interaction. However, carbonates have not yet been conclusively identified on the surface of Mars, making the search for carbonates within the dust from a regolith sample an important component for detailed mineralogical study. Microscopic examination of the regolith sample in terrestrial laboratories would enable micrometeorites to be identified from which meteorite fluxes could be estimated. A regolith sample is also likely to retain some CO2 and H2O. These might occur as ice or mixed clathrates. If acquired samples could be refrigerated at -10 to -20C, it might be possible to identify their various potential species. Determination of CO2 and H2O abundance and isotopic compositions would lead to a greater understanding of the global inventories and cycling between crust, atmosphere and poles of these compounds. For example, accurate paleotemperatures of hydrothermal systems could be determined from measurements of 18O/16O isotopic fractionation during water-mineral isotopic exchange in hydrothermal assemblages (sampled across Mars or in meteorites) using the isotopic analyses of Martian ice as the starting water reservoir composition (Bridges et al. 2001, Valley et al. 1997). If a polar landing is not chosen then the regolith sample would take on additional importance as a likely source of the ice. It is important to note that for a geologic unit with a high presumed degree of heterogeneity, like the Martian regolith, many of the measurements of interest could (and should) be done in situ, and regolith studies should be an important target for both landed missions and MSR. The basic field relationships, including measuring physical properties and their variation vertically and laterally, would best be done in place. However, sample return would be the best way to identify the altered and partially altered materials, trace minerals (e.g., carbonates), rare lithologies, etc. It is also important to note that our experience with the Spirit rover has shown us that we don’t really have a good way of knowing the magnitude of geochemical/geologic variability within this unit on a planetary scale, and how many samples will be necessary to characterize it. This objective should be thought of as one that would require more than just the first MSR mission. FINDING. The regolith is an important part of the Martian geologic system. Understanding how it was formed and modified, how and why it varies from place to place, and the role it plays in the water and dust cycles would be an important component of sample return. |