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From: American Astronomical Scoiety, Division of Planetary Science, October, 1995 meeting in Hawaii.

Mars: Mineralogical Constraints and Comparison to the Earth

R. N. Clark (USGS, Denver) and J. B. Dalton (U. C. Boulder)

Terrestrial materials have long served as analogs of the Martian surface, both chemically and spectrally. Unfortunately, we only have spectra of large areas of Mars (200+ km diameter areas) and spectra of only small hand samples of soils and minerals on the Earth. However, we now have calibrated AVIRIS data for about 1,100 sq. km of California, Nevada, and Utah, approaching the sampling area of spots on Mars whose spectra have been measured from Earth-based telescopes. Surface mineralogy in these areas has been mapped and includes clays such as montmorillonite, kaolinite, halloysite, and dickite, sulfates like alunite and jarosite, iron oxides, including hematite, goethite, nanocrystalline hematite, amorphous iron oxides, carbonates including calcite and dolomite, and a host of other minerals, including chlorites, pyrophyllite, chalcedonay, muscovites, epidotes and others. Remarkably, the average terrestrial spectrum shows overall spectral shape and albedo level to similar spectra of martian bright regions and some smaller areas, containing nanocrystalline hematite, match spectra of Mars rather well in the visible portion of the spectrum. There are important differences in the terrestrial spectra, however. The terrestrial spectra show strong clay absorptions. Despite the large variety of minerals and their spectral signatures, the 1,100 sq. km average is spectrally dominated by an absorption center at 2.206 microns having a band depth of 4.6%, and a FWHM of 0.046 microns. If this spectrum were interpreted as we interpret martian spectra, we would conclude that the arid western U.S. is composed of hematite (mostly nanocrystalline hematite) and clay minerals consisting of a mixture of montmorillonite (possibly with trace muscovite) plus halloysite and/or kaolinite and a small amount of carbonate. We would estimate the clay abundance, based on laboratory mixture modeling at about 20 wt%, consistent with typical soils on the Earth. The strength of comparable features (at 2.2 microns) in spectra of Mars are about 50 times less deep, implying very low global abundance of OH-bearing materials with 2.2-microns absorptions. The globally averaged clay abundance (e.g. montmorillonite and kaolinite group minerals) on Mars is very low and implies a very different geologic weathering history than on the Earth.



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