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E. Lellouch, T. Encrenaz (Obs. Meudon), P. Morris (Univ. Amsterdam), T. de Graauw (Sron, Groningen), S. Erard (IAS, Orsay), J. C. Pearl (NASA/GSFC)
In July--August 1997, the Infrared Space Observatory (ISO) acquired three 2.4--45 \mum spectra of Mars at R = 1500--2500 resolution. The spectra clearly show the resolved CO2, H2O and CO which can be modelled and divided out. This makes appear spectral mineralogic features in the 5--12 \mum range, originating either in the suspended dust or at the surface. Besides the prominent broad silicate absorption at 8--12 \mum, the spectra show weaker absorptions around 5.7, 6.3 (tentative), 7.2, and 11.2~\mum. \\ Reassessment of earlier observations suggests that these bands, and in particular those at 7 and 11 \mum, have actually been detected before, although not much attention has given to them. A 7 \mum feature was reported by Pollack et al (JGR 1990) from KAO observations, correctly identified as partly gaseous and partly mineralogic, and tentatively assigned to carbonate / bicarbonate ions. A non--gaseous band at 7.2 \mum is also clearly present in a large average of Mariner 9 /IRIS data (see Maguire, Icarus 1977), in some Mariner 6/7 IRS spectra (see Kirkland et al. LPSC 1998), and in MGS/TES observations (see Christensen et al. Science 1998). After elimination of the gas contribution, the Mariner 9 spectrum exhibits a subtle absorption at 11.0--11.5 \mum, though not as narrow as in the ISO data. A 11.2 \mum shoulder is also present is ground-based spectra of Acidalia (Moersch et al. Icarus, 1997). \\ Association of bands near 7 and 11 \mum may be a hint for the presence of carbonates (see companion talk by Morris et al.). We will investigate the variation of these features in the Mariner 9 spectra. Finally, we will present an analysis of the ISO spectrum longward of 18 \mum. Although the ISO spectra are affected by non--linear photometric response there, an indirect method based on the water vapor rotational features allows us to infer the surface/aerosol emissivity, and a 31 \mum absorption feature is tentatively suggested.