Spectral evidence for the mineralogy of high‐albedo soils and dust on Mars

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Abstract
Because of the lack of direct mineralogic data available for Mars, spectral remote sensing techniques and, in particular, earth‐based reflectance spectroscopy remain the primary source of this information. Presented here are laboratory observations which further constrain the mineralogy and origin of the high‐albedo soils and dust. Earth‐based observations show the reflectance spectra of classical bright regions to be fairly uniform, with a strong Fe3+ → O2− charge transfer absorption edge extending from the near UV through the visible. This absorption is relatively smooth, unlike those observed for crystalline ferric oxides which have superimposed Fe3+ crystal field bands. Dilution of ferric oxide with a spectrally more neutral medium (montmorillonite) weakens the charge transfer and crystal field absorptions together and does not serve to make the ferric oxide spectrum more Mars‐like (smoother). Nontronite (ferric‐iron‐bearing smectite clay) was also investigated for spectral agreement with telescopic observations. Pure nontronite has Fe3+ absorptions analogous to but different from ferric oxides. As in the previous case, the absorptions are very distinct and inappropriate for Mars. Admixture of neither montmorillonite nor ferric oxide serves to improve the nontronite spectrum. The conclusion is that nontronite is not a major component of martian soils, although the presence of other iron‐poor clays cannot be totally excluded based on currently available observational data. The best spectral analogs known for high‐albedo soil and dust are a specific type of palagonite from Hawaii: X‐ray amorphous weathering products of mafic volcanic glass. The indication is therefore that ferric iron on Mars is likely to occur in poorly defined crystallo‐graphic sites such as found in these amorphous materials. These materials form slowly under semiarid conditions at ambient temperature. Even low‐temperature hydrothermal alteration of glass might provide enough ion mobility to favor some formation of crystalline clays over amorphous gels and may therefore not be the primary mechanism responsible for soil formation on Mars. The amorphous Hawaiian soils exist metastably for thousands of years on Earth. Similar materials on Mars would be expected to survive considerably longer under the present cold and dry climatic conditions.