THE SURFACE OF SYRTIS MAJOR - COMPOSITION OF THE VOLCANIC SUBSTRATE AND MIXING WITH ALTERED DUST AND SOIL

Syrtis Major is an old, low relief volcanic plateau near the equatoria l regions of Mars. It is a persistent low-albedo feature on the planet and is thought to contain a high abundance of exposed bedrock and/or locally derived surface material and debris. Spatially resolved variat ions in surface spectral properties, and therefore composition, are in vestigated with data from the Imaging Spectrometer for Mars (ISM) inst rument. ISM obtained 128 wavelength channel spectrum from 0.76 to 3.16 mum for contiguous pixels approximately 22 x 22 km in size across muc h of the plateau. The value and spatial distribution of four primary s pectral variables (albedo, continuum slope, wavelength of the ferric-f errous band minimum, area of the ferric-ferrous absorption) are mapped and coregistered to Viking digital photomosaics. Analysis of these ma ps shows that although there is a high degree of overall spectral vari ability on the plateau, the key indicators of mafic mineralogy are rel atively homogeneous. Detailed examination of reflectance spectra from representative areas across the plateau indicate the volcanic surface is dominated by augite-bearing basalts and the pyroxene composition in the basalts is estimated to be 0.275+/- 0.075 Ca/Ca+Fe+Mg) and 0.3+/- 0.1 Fe/(Fe+Ca+Mg). Additional mineral components may include olivine, feldspar, and glass. Most of the spectral variability on the plateau is interpreted to result from mixing of volcanic bedrock and/or locall y derived surface material and debris with highly altered dust and soi l. In western Syrtis Major the altered material is a transient compone nt on the surface or occurs in large spatially coherent patches (e.g., crater rims). In eastern Syrtis Major it is apparent that the dust co mponents are firmly fixed to the basaltic substrate as a stable oxide rind or coating.