Evidence for a bimodal size distribution for the suspended aerosol particles on Mars

First simultaneous analysis of the ultraviolet (UV) and infrared (IR) atmospheric extinctions from SPICAM/Mars Express solar occultations in the beginning of the Northern summer (Ls = 56–97°) is presented. The two SPICAM channels allow sounding of the martian atmosphere in the spectral range from 0.118 to 1.7 μm at the altitudes from 10 to 80 km. Based on Mie scattering theory with adequate refraction indices for dust and H2O ice, a bimodal distribution of aerosol has been inferred from the SPICAM measurements. The coarser mode is represented by both dust and H2O particles with average radius of 0.7 and 1.2 μm, respectively, with number density from 0.01 to 10 particles in cm3. Clouds belonging to the aphelion cloud belt have been observed in midlatitudes in the Southern and the Northern hemispheres at altitudes of 20–30 km. The clouds are formed of large particles, and their opacity in the UV and the IR is below 0.03. The finer mode with a radius of 0.04–0.07 μm and a number density from 1 cm−3 at 60 km to 1000 cm−3 at 20 km has been detected in both hemispheres. In the Southern hemisphere the finer mode extends up to 70 km, whereas in the Northern hemisphere it is confined below 30–40 km. The lack of condensation nuclei is consistent, but could not fully explain the high water supersaturation observed between 30 and 50 km in the same Northern hemisphere dataset (Maltagliati L., Montmessin, F., Fedorova, A., Korablev, O., Forget, F., Bertaux, J.-L. [2011]. Science 333, 1868–1871). The average size of the fine mode (∼50 nm) and the large number density (up to 1000 cm−3) most likely corresponds to Aitken particles (r < 0.1 μm). This mode is unstable against coagulation and requires a continuous source of particles to be maintained, at least one order of magnitude more than estimations for the meteoric flux. A possible source is the dust lifting from the surface and dust devils. A detailed microphysical modeling is required to study the probability of survival of the observed bimodal distribution.