THE FERRIC MINERALOGY OF MARS

Abstract

ABSTRACT This dissertation presents the results of new telescope observations of Mars using the technique of imaging spectroscopy. Data at high spectral resolution (lambda/delta-lambda=350) and at the best possible spatial resolution from Earth (80-150 km) were obtained from Mauna Kea Observatory during the 1988 perihelic opposition. Spectra in the 0.4-0.8 micron region reveal distinct absorption features and spectral slope changes that are characteristic of Fe3+ bearing minerals. Poorly crystalline materials, similar perhaps to nanophase ferric minerals or palagonitelike weathering products of basaltic glass, dominate the spectral behavior of the Martian surface in the visible to near IR. Analysis of absorption-band shapes and positions and the character of the strong near-UV ferric absorption edge provides solid evidence for the detection of minor amounts (4-8%) of crystalline hematite (alpha-Fe2O3) on Mars. While there is no unique evidence in the 0.40-0.95 micron region for the existence of other ferric oxides/oxyhydroxides, Fe-rich clays, or ferric sulfates, in these new data or in previous spacecraft and telescopic data, the existence of these and other ferric-bearing phases (e.g., goethite, jarosite, ferrihydrite, feroxyhite, maghemite) cannot be unequivocally ruled out, partly because of the spectral masking effects of hematite. Different models for the formation of hematite and other ferric minerals in various terrestrial analog environments and in the current and possibly past warmer, wetter Martian climate are discussed. Images in the 0.4-1.0 micron region reveal the ''classical'' albedo features at red and green wavelengths (lambda > 0.5 microns) and show a spectrally bland surface dominated by polar ices and atmospheric condensates at blue wavelengths. A number of differnet telescopic laboratory data analysis techniques are used to show that (1) the 2%-5% deep 0.6-0.7 micron ferric absorption bands varies across the surface at the 1%-2% level, with bright regions typically having a deeper band; (2) many dark regions and a few isolated bright regions are perhaps more spectrally hteterogeneous than once thought; (3) 95% of the variance in Mars spectra can be modeled using two endmembers (classical bright and dark regions), but there are distinct spatially coherent units within the remaining 5% of the variance that correlate with ices, condensates, and/or dark, ferric-rich materials; (4) numerous ferric minerals have absorption features in the 0.9-1.0 micron region, and the weak bands observed in previous Mars spectra at these wavelengths that have been ascribed entirely to Fe2+ minerals may, within the limits of the available data, also be consistent with variations in Fe3+ mineralogy. The advantages of imaging spectroscopy over traditional point spectroscopy or broadband filter imaging make it an ideal tool for high spatial-resolution spacecraft studies of the Martian surface.