POST-PHOBOS MODEL FOR THE ALTITUDE AND SIZE DISTRIBUTION OF DUST IN THE LOW MARTIAN ATMOSPHERE

Four experiments flown on board Phobos 2 provided information on the characteristics of the dust particles suspended in the Martian atmosphere : Auguste (UV-visible-IR spectrometer working in solar occultation geometry), ISM (IR spectrometer measuring the light of the Sun reflected by the planet), Termoskan (scanning radiometer mapping the planetary thermal radiation), KRFM (UV-visible multiphotometer providing limb-to-limb profiles). These experiments, which sounded equatorial legions (20 degrees S-20 degrees N) near the northern spring equinox (L(S)=0-20 degrees), are shown to yield a reasonably consistent picture of the dust distribution over the whole altitude range from the ground level, or just above, outside the boundary layer, up to approximate to 25 km. The vertical profiles of particle volume mixing ratio and effective (projected area-weighted) radius deduced from Auguste measurements, performed in the 15-25 km altitude range, are extrapolated down to the ground by using a simple, physical parameterization of the altitude dependence of dust mixing ratio and radius. This parameterization, which must be understood as describing the vertical distribution of dust in the zonal average and on the mesoscale in latitude, assumes that gravitational settling and eddy diffusion are the only two processes driving vertical dust transport. The vertically averaged effective radius and optical depth of dust particles, as well as vertical profiles of related quantities, are obtained. Optical depth at 1.9-mu m wavelength is found to be 0.2 on average, with a typical variation of +/-0.1 with time and space. This result is similar to that obtained from ISM spectra analysis. It is also consistent with the Termoskan and KRFM measurements, which yield near-infrared optical depths of 0.12-0.26 and 0.12-0.24, respectively. The particle number density near the surface, as derived from extrapolation of solar occultation profiles, is in the range 1-3 cm(-3), in good agreement with Termoskan results (1-2 cm(-3)). The scale height of the dust volume mixing ratio just above the surface is approximate to 8-9 km on average, that is, of the same order as the background atmospheric scale height. The vertically averaged effective radius of dust particles is found to lie in the range 1.7+/-0.2 mu m, possibly approximate to 2 mu m in the case of a large effective variance of 0.4. The most likely ISM value is 1.2 mu m, with a rather large uncertainty of +/-0.4 mu m, mainly due to the fact that the spectral dependence of the Minnaert coefficient is not well known. Because ISM data used in the present work were obtained on the Tharsis plateau, at a mean altitude of approximate to 7 km, the ISM radius must be compared to the Auguste vertically averaged radius for z>7 km, that is, approximate to 1.5+/-0.2 mu m. Auguste and ISM radii are therefore consistent at the 1-sigma level. Three typical vertical profiles of the dust particle radius and number density, obtained by averaging all solar occultation profiles, including their extrapolated parts below approximate to 15 km, are proposed as reference models, for three selected values of the effective variance of the particle size distribution (0.10, 0.25, and 0.40).