V.E.Regolith - Science Priorities for Mars Sample Return


V.E.Regolith
The 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 CO V.E.Regolith - Science Priorities for Mars Sample Return2 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return-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 V.E.Regolith - Science Priorities for Mars Sample Return., 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).

The mixed and complex nature of regolith samples could V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return retain some CO2 and H2O. These might occur as ice or mixed clathrates. If acquired samples could be refrigerated at -10 to -20C, it might be possible to identify their various potential species. Determination V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return), 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 V.E.Regolith - Science Priorities for Mars Sample Return magnitude of geochemical/geologic variability within this unit on a planetary scale, and how many samples would be necessary to characterize it. This objective should be thought of as one that would require V.E.Regolith - Science Priorities for Mars Sample Return 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 V.E.Regolith - Science Priorities for Mars Sample Return place to place, and the role it plays in the water and dust cycles an important component of sample return.



^ V.F.Polar Ice
Samples of polar ice would be necessary V.E.Regolith - Science Priorities for Mars Sample Return to constrain the present and past climatic conditions, as well as elucidate cycling of water, on Mars. The samples necessary to achieve these objectives could include discreet samples of surface ice from the V.E.Regolith - Science Priorities for Mars Sample Return Polar Layered Deposits (PLD) or a seasonal frost deposit. Short cores (~1 cm diameter x 30 cm length) from the PLD or subsurface ice deposit would also be desirable. A single sample could provide critical V.E.Regolith - Science Priorities for Mars Sample Return input on surface/atmosphere interactions. A short core might resolve climate variability in the last few 100 Ka to 1 Ma [Milkovich and Head, 2005]. Annual layers could be observed in core samples and isotopic V.E.Regolith - Science Priorities for Mars Sample Return signatures (18O, D/H) are expected to define annual temperature variability, changes in water reservoir availability and exchange with the atmosphere, and short-term climate variations (Fisher, 2007). The composition of V.E.Regolith - Science Priorities for Mars Sample Return entrained non-ice dust materials (e.g., aeolian, volcanic tephra, impact glass) would help determine the sources and relative proportions of dust reaching the poles.  Changes in the amount of entrained non-ice V.E.Regolith - Science Priorities for Mars Sample Return dust with depth would help to constrain estimates of the modulation of large-scale dust events and their seasonal variability (Herkenhoff et al., 2007). The desired sample localities would include both north and south V.E.Regolith - Science Priorities for Mars Sample Return residual ice deposits, both north and south PLD, and both mid-latitude and tropical glacial deposits (Head et al., 2006; Head and Marchant, 2003; Shean et al., 2005; Shean et al., 2007). Ideally several V.E.Regolith - Science Priorities for Mars Sample Return core samples would be extracted over lateral distances of ~1 km to validate stratigraphic models based on orbital imagery. On the polar plateaus, the areas between scarps and troughs are wide and V.E.Regolith - Science Priorities for Mars Sample Return flat, and the north polar troughs have walls whose maximum slopes are ~10°. A traverse that would acquire multiple discreet samples along trough slopes where stratigraphy is well exposed would afford extensive vertical sampling of V.E.Regolith - Science Priorities for Mars Sample Return climate history (Carsey et al., 2005). Trough slopes are well within the range of slopes that the Mars Exploration Rovers successfully traversed in Endurance and Victoria craters and the Columbia Hills.

Either V.E.Regolith - Science Priorities for Mars Sample Return drilling or coring technologies would be required to sample the ice. The capability to acquire 30 cm cores is not expected to require significant technology development. Technologies for coring or small drills exist from MSL V.E.Regolith - Science Priorities for Mars Sample Return and have been proposed for Scout missions. Scooping or drilling would be required to sample surface ice or ice buried under dry soil. These samples must be encapsulated and kept frozen; however V.E.Regolith - Science Priorities for Mars Sample Return, melt water would still provide critical isotopic and compositional information. Dividing cores into sub-samples is expected to be similar to that for rock samples but it must be conducted V.E.Regolith - Science Priorities for Mars Sample Return under controlled conditions. Stratigraphic analyses of the cores must be conducted before they are divided and, if sub-samples are accurately catalogued, the core could be returned to Earth in sections.


FINDING V.E.Regolith - Science Priorities for Mars Sample Return. A single ice sample could provide critical input on surface/atmosphere interactions. A carefully selected short core might resolve climate variability during the last few 105 to 106 years. Although ND-SAG recognizes that V.E.Regolith - Science Priorities for Mars Sample Return returning an ice sample on the first MSR would be implausible, it is important to keep this sample type in mind for future MSRs.




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