Observations of the Martian sky, Phobos, and the sun were taken with the Viking lander imaging cameras to obtain information on the properties of the atmospheric aerosols. Atmospheric optical depths were derived from the observations of the brightness of the celestial objects. Information on the absorption coefficient, mean size, and shape of the aerosols was derived from studies of the sky brightness. For this purpose we used a multiple-scattering computer code that employed a recently developed technique for treating scattering by nonspherical particles. By monitoring the brightness of the twilight sky we obtained information on the vertical distribution of the particles. Three types of aerosols are inferred to have been present over the landers during the summer and fall season in their hemisphere. A ground fog made of water ice particles was present throughout this period. It formed late at night during the summer season and dissipated during the morning. We infer that during the summer the frost point temperature was 195°K and the water vapor volume mixing ratio equaled about 1× 0−4 near the ground at VL-2. Assuming that condensation occurs only on suspended soil particles, we estimate that the average particle radius of the fog was about 2 μm and that the fog's depth equaled approximately 0.4 km. A higher-level ice cloud was prominent only during the fall season, when it was a sporadic source of atmospheric opacity at VL-2. The formation of upper level water ice clouds during the summer may have been inhibited by dust heating of the atmosphere. Suspended soil particles were present throughout the period of observation. During the summer they constituted the only major source of opacity in the afternoon and most of the night. The cross-section weighted mean radius of these aerosols is about 0.4 μm. They have a nonspherical but equidimensional shape and rough surfaces. These soil particles have a scale height of about 10 km, which is comparable to the gas scale height, and they extend to an altitude of at least 30 km. The principal opaque mineral in these particles is magnetite, which constitutes 10%±5% by volume of this material. We propose that soil particles, as well as any associated water ice, are eliminated from the atmosphere, in part, by their acting as condensation sites for the growth of CO2 ice particles in the winter polar regions. The resultant CO2-H2O-dust particle is much larger and therefore has a much higher fallout velocity than an uncoated dust or water ice particle.
Aircraft and Space Shuttles flying through the stratosphere over the next several decades will add sulfuric acid and aluminum oxide particles, respectively, to this region of the atmosphere. To evaluate the effect of these additional aerosols on the global heat balance, we have performed solar and terrestrial radiative transfer calculations. The solar calculations employed an accurate numerical method for solving the multiple-scattering problem for unpolarized light to determine the dependence of the global (spherical) albedo on the optical depth perturbation Δτ. Correct allowance was made for absorption by gases. Using these results, and those obtained from calculations of the terrestrial thermal flux at the top of the atmosphere, we determined the resulting change in the mean surface temperature, ΔT, as a function of Δτ. In both calculations, we used the measured optical constants of the aerosol species. To apply these results to the problem of interest, we used engine exhaust properties of the various types of vehicles to estimate their optical depth perturbation and examined the record of past climate changes to set a threshold value, 0.1 K, on the mean surface temperature change, below which no significant impact is to be expected. Using the above information, we find that no significant climate change should result from the aerosols produced by Space Shuttles, SST's, and other high flying aircraft, operating at traffic levels projected for the next several decades. However, the effect of SST's is sufficiently close to our threshold limit to warrant a reevaluation as their characteristics are updated.
The Arakawa‐Mintz general circulation model of the Earth has been applied to Mars at the season of the nominal Viking Lander mission. Allowance has been made for the effects of the large Martian topography. The calculated wind fields exhibit significant zonally symmetric, topographically forced, and diurnal tidal components. Average wind speeds of 20‐25 m/sec were found at three possible landing sites, with wind speeds occasionally reaching values as much as a factor of 2 larger than these average values. If buffering by CO 2 adsorbed in the regolith can be neglected, a pressure decline of about 0.8 mb is expected over the first 2 months of operation of the first lander.
