Martian winter atmosphere at north high latitudes: Mariner 9 IRIS data revisited Original Research Article

Several hundred spectra were obtained at latitudes > 65N in late northern winter (Ls = 322° −352°). Solar zenith angles ranged from 75° to 125°; however most of these spectra were observed in polar darkness. The data reveal complex spectral features, including a complex shape in the 15 μm CO2 band. Temperature profiles and aerosol opacities were retrieved in a self-consistent way. Temperature profiles show an inversion in the lowest 5 km above the local surface(below 0 km altitude). At higher levels the temperature decreases with altitude, reaching a minimum at 10 – 20 km, in such a way that the altitude of the minimum increases with latitude; above that level the temperature increases again up to 40–50 km. At latitudes ϕ <70 N the temperature in the atmosphere above the inversion is too high for CO2 condensation, but at 70–75 N in some cases the CO2 condensation occurs at the altitudes below 5 km. The maximal altitude of the possible CO2 condensation increases up to 20 km in the near polar region at 80–90 N. A near surface fog, consisting of the H2O ice particles, was found with typical values of the optical depth at 10 μm ranging from 0.1 to higher than 1. For an exponential model, its scale height was found to be 1–2 km, with a mean particle radius of about 1 μm (assuming a normal particle size distribution). Total water content in the fog is 1–10 prμm (10 – 100 ppm at −5 km surface altitude). Most of the spectral features in the range 200 – 1000 cm−1 are successfully reproduced by our model, except the spectra of anomalously cold areas, where more sophisticated model is needed, which should include the CO2 surface deposits and condensation clouds near the surface. Most of the spectra with these features are observed at latitudes > 80N and in some cold areas at 70–80N.