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[75.04] Sounding the Martian dust from ISO spectroscopy in the 2.7 \mum CO₂ bands

D.V. Titov (MPIAe, Lindau), A.A. Fedorova (IKI, Moscow), E. Lellouch (Obs. Meudon), T. de Graauw (Sron, Groningen), H. Feuchtgruber (MPIEP, Garching)

In July--August 1997, the Infrared Space Observatory (ISO) acquired three 2.4--45 \mum spectra of Mars at 1500--2500 spectral resolution. Two out of the three spectra show the presence of weak but non--zero flux level (›(5--8)\times10^-3 erg cm^-2s^-1sr^-1/cm^-1)) in the center of the › \nu₁+\nu₃ and 2\nu₂+\nu₃ bands of CO₂ at 2.69 and 2.77 \mum. This is evidence for aerosol scattering, as models without dust predict zero flux in these saturated bands. Calculations by Titov, Fedorova and Haus (Planet. Space Sci., in press) show that the spectrum at 2.65--2.80 \mum can be used to determine the aerosol optical depth and estimate its vertical distribution. A preliminary examination of the ISO data suggests an optical depth of about 0.1. More refined estimates will be presented, and compared with concommitant (though local) Mars Pathfinder results at visible wavelengths.

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