VIKING LANDER IMAGE-ANALYSIS OF MARTIAN ATMOSPHERIC DUST

We have reanalyzed three sets of Viking Lander 1 and 2 (VL1 and VL2) i mages of the Martian atmosphere to better evaluate the radiative prope rties of the atmospheric dust particles. The properties of interest ar e the first two moments of the size distribution, the single-scatterin g albedo, the dust single-scattering phase function, and the imaginary index of refraction. These properties provide a good definition of th e influence that the atmospheric dust has on heating of the atmosphere . Our analysis represents a significant improvement over past analyses (Pollack et al. 1977, 1979) by deriving more accurate brightnesses cl oser to the sun, by carrying out more precise analyses of the data to acquire the quantities of interest, and by using a better representati on of scattering by nonspherical particles. The improvements allow us to better define the diffraction peak and hence the size distribution of the particles. For a lognormal particle size distribution, the firs t two moments of the size distribution, weighted by the geometric cros s section, are found. The geometric cross-section weighted mean radius (r(eff)) is found to be 1.85 +/- 0.3 mu m at VL2 during northern summ er when dust loading was low and 1.52 +/- 0.3 mu m at VL1 during the f irst dust storm. In both cases the best cross-section weighted mean va riance(v(eff)) of the size distribution is equal to 0.5 +/- 0.2 mu m. The changes in size distribution, and thus radiative properties, do no t represent a substantial change in solar energy deposition in the atm osphere over the Pollack et al. (1977, 1979) estimates.