Which spectral range is the most promising one for carbonate detection on Mars?

Abstract

In spite of extensive search carbonates have not been found until nowon the Martian surface by spectroscopic methods. This is surprising because aa carbonate content up to 5 or even 10 % is suggested by (i) theoretical conceptions about the weathering processes of Martian crustal rocks, and (ii( further arguments, e.g. the Viking chemical analyses of Martian soil. But also the recent earth-based Mars spectra, covering the wavelength range 2-4.2 m (Blaney and McCord 1989, Roush et al. 1992), provided no spectral evidence for carbonates in the Martian soil. This could be due to the fact that the diagnostic feature at 4 muem, which is a trough in the reflectance spectrum, is paritally filled by Mars' thermal emission (Wagner and Schade, 1996). In the present laboratory study it is investigated, whether a better chance for carbonate detection may be exspected for the pure emission spectrum in the thermal infrared, i.e., by using the carbonates' fundamental vibration bands (at e.g. about 7, 11.5 and 13.5 m). For this purpose spectra were measured for mixtured consisting of palagonite (as a Mars soil analogue) with various percentages of carbonates. The band depths of the diagnostic featureswere estimated and compared with those obtained previously for the 4-m feature.