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 c haracteristics of the dust particles suspended in the Martian atmosphe re : Auguste (UV-visible-IR spectrometer working in solar occultation geometry), ISM (IR spectrometer measuring the light of the Sun reflect ed by the planet), Termoskan (scanning radiometer mapping the planetar y 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 k m. The vertical profiles of particle volume mixing ratio and effective (projected area-weighted) radius deduced from Auguste measurements, p erformed in the 15-25 km altitude range, are extrapolated down to the ground by using a simple, physical parameterization of the altitude de pendence 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 grav itational settling and eddy diffusion are the only two processes drivi ng vertical dust transport. The vertically averaged effective radius a nd optical depth of dust particles, as well as vertical profiles of re lated quantities, are obtained. Optical depth at 1.9-mu m wavelength i s 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 spect ra analysis. It is also consistent with the Termoskan and KRFM measure ments, 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 d erived from extrapolation of solar occultation profiles, is in the ran ge 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 surfac e is approximate to 8-9 km on average, that is, of the same order as t he background atmospheric scale height. The vertically averaged effect ive 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 va riance of 0.4. The most likely ISM value is 1.2 mu m, with a rather la rge uncertainty of +/-0.4 mu m, mainly due to the fact that the spectr al dependence of the Minnaert coefficient is not well known. Because I SM 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 com pared to the Auguste vertically averaged radius for z>7 km, that is, a pproximate to 1.5+/-0.2 mu m. Auguste and ISM radii are therefore cons istent at the 1-sigma level. Three typical vertical profiles of the du st particle radius and number density, obtained by averaging all solar occultation profiles, including their extrapolated parts below approx imate to 15 km, are proposed as reference models, for three selected v alues of the effective variance of the particle size distribution (0.1 0, 0.25, and 0.40).