INDICATIONS OF SULFATE MINERALS IN THE MARTIAN SOIL FROM EARTHBASED SPECTROSCOPY

Telescopic measurements of Mars between 4.40 and 5.13 mu m at a spectr al resolution (lambda/Delta lambda) of 300 were made on August 19, 198 8, UT at the NASA infrared Telescope Facility on Mauna Kea, Hawaii. Th is wavelength region contains radiation from both solar reflection and thermal emission. Additionally, the Martian atmosphere has numerous a tmospheric gas absorption features, dominated by the 4.2-mu m CO2 fund amental band. The Mars spectrum rise out of the 4.2 to 4.4-mu m CO2 ba nd on the long-wavelength side cannot be matched solely by atmospheric gas constituents. An absorption must be added at roughly 4.5 mu m in order to decrease the reflectance rise and produce the 4.5-mu m inflec tion which is present in the data. The location of this feature at the position of the 2 nu 3 overtone of the SO42- anion indicates that the surface absorption is probably caused by sulfates on the Martian surf ace and/or in atmospheric dust, This is consistent with the known pres ence of sulfates on the Martian surface from the Viking results. An ex act spectral match to a terrestrial sulfur mineral has not been made, but we suggest that the mineral on Mars has very weak band structure, probably due to an ion environment in the mineral with a high degree o f electric field symmetry. Significant variation exists at 4.5 mu m am ong the observed spectra for different locations on Mars. In order of strength, from strongest absorption to weakest, are Eastern Solis Plan um, Argyre Basin, Eastern Tharsis, and Valles Marineris for the four r egions measured at similar Mars atmospheric conditions.