The Martian atmosphere in the region of the great volcanoes: Mariner 9 IRIS data revisited

The structure of the Martian atmosphere in the region of the Great Volcanoes has been investigated using the Mariner 9 IRIS data. Altogether, 337 spectra were studied; 334 of them were obtained in the afternoon between 13 and 18 h in the range Ls=314–348° with footprints less than 200 km. The remaining 3 spectra, which reveal pronounced water ice features, were obtained at Ls=98° near 16 h. All spectra were corrected for the recently discovered instrumental effect (Formisano et al. (Planet. Space Sci. 48 (2000) 569). It is shown that neglecting it would lead to an overestimate of the atmospheric temperature. Temperature profiles and aerosol opacities were retrieved in a self-consistent way. It is found that the aerosol opacity varies in the range 0.1<τa<0.8, with an error of 30%. The data were obtained during the decay of the dust storm. During the more dusty period (Ls=314–316°) the aerosol opacity tends to increase with local time in the afternoon. We consider three paths through Tharsis, taken during this period. For late afternoon conditions, the magnitude of the inversion in the temperature profile increases with local time. It becomes especially pronounced (Tmax–Tsurf>20 K) after 17 h, when the solar zenith angle exceeds 80°. The surface temperature at the top of Arsia Mons changes by up to 30 K within one hour after 17 h, while the atmospheric temperature varies only by several degrees. We find that the atmospheric temperature above the top of Arsia has decreased by about 30 K between Ls=314° and 347°, whereas the aerosol opacity decreased from 0.3 to 0.1. In the latter case, water condensation occurs and, hence, aerosols are composed of dust and water ice. It is found that about 0.1 prμm of H2O ice needs to be included into the aerosol component to account for its spectral behavior in the continuum. The water ice clouds were observed near Martian aphelion (Ls=98°) at low Northern latitudes over Pavonis and Ascraeus Mons in the afternoon. Their visual opacity is about 0.46–0.48 (averaged over the FOV) and the mean particle size is found to be Rm=2– μm (assuming a normal particle size distribution). A comparison between the temperature profile obtained at Ls=98° and that derived for the same location and local time (15:40) at Ls=348°, shows that the atmospheric temperature near aphelion is about 20 K lower.