Charge-coupled device imaging spectroscopy of Mars. 2. Results and implications for Martian Ferric mineralogy

Imaging spectroscopic observations of Mars in the visible to near-IR (0.4-1.0 μm) were conducted during the 1988 opposition from Mauna Kea Observatory. Data at high spectral resolution (λ/Δλ = 350) and at the best possible spatial resolution from Earth (80-150 km) reveal distinct absorption features and spectral slope changes that are characteristic of Fe³⁺-bearing minerals. Poorly crystalline materials, similar perhaps to nanophase ferric oxides or palagonite-like weathering products of basaltic glass, dominate the spectral behavior of the Martian surface in the visible to near-IR. Analysis of spectral shape and absorption band positions provides evidence for the detection of minor amounts (perhaps 4-8%) of crystalline hematite (αFe₂O₃) on Mars. While there is no unique evidence in the 0.40- to 1.0-μm region of other ferric oxides/ oxyhydroxides, Fe-rich clay silicates, or ferric sulfates in these new data or in previous spacecraft and telescopic data, the existence of these phases cannot be unequivocally ruled out, partly because of the spectral masking effects of hematite. Models for the formation of hematite and other ferric minerals in various terresterial environments and in the current and possibly past warmer, wetter Martian climate are discussed. Telescopic and laboratory data analysis techniques are used to show that (1) the 2-5% deep 0.6- to 0.7-μm ferric absorption band 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 heterogeneous than once thought; (3) 95% of the variance in Mars spectra can be modeled using two linear spectral 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, ferricrich materials; and (4) numerous ferric minerals have absorption features in the 0.9- to 1.0-μm-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 and telescopic studies of the Martian surface.