Iron-montmorillonite: a spectral analog of Martian soil

Preparation, characterization and sorption properties for phosphates of hematite, bentonite and bentonite-hematite systems

Hematite was prepared using Fe(NO3)3*9H2O. Bentonite was first saturated with sodium and then was transformed to the hydrogen form. The bentonite-hematite (b-h) system was prepared by the mixing of hematite and bentonite at pH 6.0, and a hematite-coated bentonite surface was created. The physical and chemical properties of the above solids were studied from the information given by X-ray, IR and NMR spectra and by measurement of specific surface and point of zero charge. The results of the above study show that the bentonite-hematite system is not a physical mixture of hematite and bentonite. Hematite moved into bentonite's interlayer space, coating these planes throughout. Consequently, Al-substitution and structural H2O/OH displacement of hematite took place. The system has a specific surface, less than bentonite and greater than hematite, while its p.z.c. is higher than bentonite and lower than hematite. Adsorption experiments of phosphates at different concentrations and pH were carried out and the constant capacitance model was used to describe phosphate adsorption by hematite, bentonite and the bentonite-hematite system. The model is characterized by a ligand exchange mechanism based on the assumption that the charge is attributed both on adsorbate and adsorbent, and is proven to successfully describe the adsorption of phosphate on these materials along with the effect of varying pH values. Ma-Za 1 and Ma-Za 2 (two computer programs) were used for this purpose. The model successfully gave a description (both quantitative and qualitative) of the adsorption of phosphate anions across the pH range (3.8-9.0). Furthermore, it can be successfully applied in systems as the bentonite-hematite system by utilizing the surface protonation-dissociation constant of hematite. The model, when applied in such systems, procreates the exact shape of the adsorption isotherms for the entire pH range of 3.8-9.0. The constant capacitance model was able to describe phosphorous adsorption on hematite and the bentonite-hematite system. The accurate fit of the model to phosphorous adsorption on hematite and the bentonite-hematite system suggests that inner sphere surface complexation is the appropriate adsorption mechanism for these materials.

Laboratory investigations of Mars: chemical and spectroscopic characteristics of a suite of clays as Mars soil analogs

Two major questions have been raised by prior explorations of Mars. Has there ever been abundant water on Mars? Why is the iron found in the Martian soil not readily seen in the reflectance spectra of the surface? The work reported here describes a model soil system of Mars Soil Analog Materials, MarSAM, with attributes which could help resolve both of these dilemmas. The first set of MarSAM consisted of a suite of variably iron/calcium-exchanged montmorillonite clays. Several properties, including chemical composition, surface-ion composition, water adsorption isotherms, and reflectance spectra, of these clays have been examined. Also, simulations of the Viking Labeled Release Experiment using the MarSAM were performed. The results of these studies show that surface iron and adsorbed water are important determinants of clay behavior as evidenced by changes in reflectance, water absorption, and clay surface reactions. Thus, these materials provide a model soil system which reasonably satisfies the constraints imposed by the Viking analyses and remote spectral observations of the Martian surface, and which offers a sink for significant amounts of water. Finally, our initial results may provide insights into the mechanisms of reactions that occur on clay surfaces as well as a more specific approach to determining the mineralogy of Martian soils.

Simulations of the Viking Gas Exchange Experiment using palagonite and Fe-rich montmorillonite as terrestrial analogs: implications for the surface composition of Mars

Simulations of the Gas Exchange Experiment (GEX), one of the Viking Lander Biology Experiments, were run using palagonite and Fe-rich montmorillonite as terrestrial analogs of the Martian soil. These terrestrial analogs were exposed to a nutrient solution of the same composition as that of the Viking Landers under humid (no contact with nutrient) and wet (intimate contact) conditions. The headspace gases in the GEX sample cell were sampled and then analyzed by gas chromatography under both humid and wet conditions. Five gases were monitored: CO2, N2, O2, Ar, and Kr. It was determined that in order to simulate the CO2 gas changes of the Viking GEX experiment, the mixture of soil analog mineral plus nutrient medium must be slightly (pH = 7.4) to moderately basic (pH = 8.7). This conclusion suggests constraints upon the composition of terrestrial analogs to the Mars soil; acidic components may be present, but the overall mixture must be basic in order to simulate the Viking GEX results.