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[51.19] Martian Northern Polar Latitudes: the Winter Atmosphere From Mariner 9 IRIS Data

L. Zasova, V. Formisano, D. Grassi, N. Igntiev, A. Maturilli (Istituto di Fisica dello Spazio Interplanetario, Rome, Italy)

Several hundred spectra were obtained at latitudes > 65N in late northern winter (L_s = 322^\circ -352^\circ ). Solar zenith angles ranged from 75^\circ to 125^\circ ; however, most of these spectra were obtained in polar darkness. The data reveal complex spectral features, including a complex shape in the 15\mu m CO₂ band. Temperature profiles and aerosol opacities were retrieved in a self consistent way. Temperature profiles show an inversion at 5-10 km above the local surface. At higher levels the temperature decreases with altitude, reaching a minimum near 20 km; above that it increases again up to 40-45 km. A near surface fog, consisting of H₂O ice particles, was found with optical depth at 10\mu m ranging from 0.1 to greater than 1 (with typical values 0.3-0.5). For an exponential model, its scale height was found to be 1-2 km, with a mean particle radius of about 1 \mu m (assuming a normal particle size distribution). Total water content in the fog is 1-10 pr \mu m. Most of the spectral features in the range 200-900 cm^-1 are successfully reproduced by this model. Condensation of CO₂ may occur at 10-25 km altitudes for latitudes > 75N. A deposit of CO₂ may exist, with CO₂ condensation clouds present, for latitudes > 80N and in some cold spots from 70-80N.