J. B. Dalton (U. C. Boulder) and R. N. Clark (USGS, Denver)
Several terrestrial minerals have been proposed as spectral analogs for Martian surface materials, and two groups in particular, the palagonites and the montmorillonites, have proven exceptionally popular. However, neither provides a perfect match to the observations, and the diagnostic 2.2-microns cation-OH vibrational absorption exhibited by most terrestrial clays has yet to be detected in spectra of the Martian surface. We have measured and modeled mixtures of 5.0, 3.0, 1.0, and 0.5 weight % montmorillonite (SAz-1) in a spectrally bland basaltic powder having a 2.2-microns albedo similar to that of Mars. The figure below shows the variation of band depth with clay content in the mixtures. For mixtures of as little as 5.0 weight % of clay, the 2.2-microns band is readily separated from the continuum, and is clearly distinguishable down to 1.0 weight %. Below 0.5 weight %, however, existing technology cannot achieve sufficient signal-to-noise to separate the 2.2-microns absorption from background. Between 5 to 0.5 weight % clay, band depths ranged from .013 to .0011 for the basalt-montmorillonite mixtures. Examination of the literature reveals that the upper limit to the 2.2-microns band depth for Martian spectra is 0.0007 micron 0.0010 (Clark, 1992, DPS Abstract), easily less than that for the 0.5 weight % clay mixture used in this study. This would imply that Martian soils of as much as 1 weight % clay can be ruled out on the basis of available spectroscopic data.
Figure: Clay absorption feature depth as a function of abundance (4k GIF)
This figure shows measured clay absorption band depth in spectra of montmorillonite-basalt mixtures and the upper limit for the clay band in spectra of Mars.
U.S. Geological Survey,
a bureau of the U.S. Department of the Interior
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Last modified November 18, 1998.