Seasonal Variation of Aerosol Vertical Distribution in the Martian Atmosphere and Detection of the Bimodal Distribution from Solar Occultations on Mars-Express
Résumé
Dust cycle is one of the most important in the Martian climate system. Martian dust, consisting mainly of mineral particles, lifts from the surface by winds and dust devils. Being active radiation, dust with water clouds is involved in the heating and cooling of different atmospheric layers, absorbing, scattering and re-radiating solar radiation in the thermal infrared. Furthermore, the dust particles are condensation nuclei for the formation of water and CO2 ice clouds.
We will present results of long-term observations of the Martian atmosphere by solar occultation technics in the near-IR range. SPICAM spectrometer on board the Mars Express spacecraft has been working on the orbit of Mars since January 2004. During the four Martian years 800 solar occultations have been performed. SPICAM spectral range allows simultaneous observations of 1.43 μm CO2 band for the atmospheric density, the 1.38 μm absorption band of water vapor to get the H2O density, and the distribution of aerosols with altitude measuring the opacity of the atmosphere in the spectral range from 1 to 1.7 μm. In the experiment, aerosol extinction profiles have been obtained at altitudes from 0 to 90 km with a vertical resolution of 2 to 10 km depending on distance to limb. We also consider their seasonal and latitudinal variations. Retrieved values of effective radius vary from 0.1 to 1.5 μm.
Special attention was paid to the summer in the northern hemisphere, where the water vapor supersaturation in the middle atmosphere has been recently discovered (up to the values of S = p/psat ~ 3–5) [1]. Simultaneous analysis of aerosol extinction in the UV and IR range at different altitudes during this period has enabled the first direct detection of a bimodal distribution of Martian dust particles with characteristic radius of 0.04–0.07 μm and 0.7–0.8 μm. The number density of small fraction varies from 103 cm-3 at 10 km to 10cm-3 at 40 km. The concentrations and the effective radius of the particles correspond to the Aitken particles in the Earth's atmosphere. Unfortunately, the spectrometer cannot determine the nature of the particles, so dust and ice particles were considered. Such concentration of small particles in the presence of a large fraction should be unstable to coagulation process, The coagulation time for obtained bimodal di stribution varies from 1 to 50 days, which requires a source of particles. If it is not the condensation phase, the particles lift from the surface. The sedimentation time varies from 100 to 1000 days (for particles 0.1 microns and 0.01 microns, respectively) at 20 km and 10 to 100 days at 40 km, and these particles may be transported by Hadley cell from the northern to the southern hemisphere in the observed period of the summer solstice in the northern hemisphere.