Characterization of an Antarctic Mars Analog Soil and Implications for Martian Weathering Processes

Terrestrial analogs can be used to gain insight into potential martian weathering processes and the role of water in the near-surface environment. We are investigating the mineralogy and chemical properties of a fine size fraction of a soil weathered from the Ferrar Dolerite. The soil was collected near Lewis Cliff in the Transantarctic Mountains. The Ferrar exhibits mineralogical similarities to martian basaltic lithologies, as represented by the shergottites [Harvey, 2001]. Production of fines from this parent rock in the cold, arid Antarctic makes the fines a promising Mars analog material. The analog soil fines have been studied with SEM/EDS, IR spectroscopy, XRD, TEM, and Mössbauer spectroscopy. XRD-derived semi-quantitative mineral abundances reveal that the Antarctic fines contain ~30% primary phases (plagioclase feldspar, pyroxenes, a small amount of quartz) and ~70% secondary phases (clays and clay-like mineraloids, zeolites, and ~50% calcium sulfates). The fines' thermal IR spectrum revealed silicate, bound water and sulfate features, consistent with the XRD-derived mineralogy. The significant amount of secondary phases present indicate that even in the Earth's coldest, driest environment, there is enough water and energy to weather some primary minerals. Atmospheric sulfate aerosols may have been important in producing the fines' abundant sulfate salts. Oxygen isotope studies of Antarctic Dry Valleys sulfates have revealed a Δ17O anomaly, which suggests the sulfates are not just from sea salt (Δ17O =0) but also from atmospheric oxidation of gaseous sulfur compounds (e.g. marine biogenic dimethylsulfide) [e.g. Bao et al., 2000]. The anomaly implies that atmospheric sulfur aerosols interact with rocks and soils in Antarctica, similar to the acid fog model for martian weathering [e.g. Banin et al. 1997]. We have obtained an average Δ17O value of +1.67±0.05‰ for the sulfates in the Antarctic fines being investigated here. This indicates that, in this Lewis Cliff soil as well, some of the sulfate in sulfate salts was contributed from atmospheric sources. Also, several clay-like aggregate particles examined contain sulfur. Iron oxides and clays in soils can absorb sulfate anions [e.g. Parfitt and Smart, 1978]. TEM work has revealed that many particles are clays or clay-like mineraloids, which exhibit a range of crystallinity and stacking disorder, or aggregates of these. While some particles were well crystalline clays or amorphous secondary products, most clay-like particles exhibited short-range order. Also, gypsum, primary minerals and particles in which secondary minerals are associated with primary minerals were observed. The presence of particles consisting of clay minerals and mineraloids of varying crystallinity and layer orientation, or aggregates of these, indicates that the Antarctic environment does not preclude significant chemical weathering, but it is consistent with limited water availability. Using the diverse dataset produced by applying multiple techniques, the characteristics of the fines support the hypothesis that chemical weathering products were produced by interaction of acidic aerosols with soils and rocks. Additional alteration by small amounts of water (e.g. thin water films) is also a likely contributor to the weathering process.