Properties of aerosols in the Martian atmosphere, as inferred from Viking Lander imaging data

Abstract

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.

Overview of the Mars Pathfinder Mission and Assessment of Landing Site Predictions

“…The sky has been a pale pink color (Fig. 1), similar to what was seen by the Viking landers (14). Particle size (roughly a micrometer) and shape and water vapor (about 10 precipitable micrometers) in the atmosphere are also consistent with measurements made by Viking (14).…”

Opacity of the Martian atmosphere measured by the Imager for Mars Pathfinder

“…Optical depths were measured throughout the Martian year by the Viking Landers at both landing sites [Pollack et al, 1977[Pollack et al, , 1979 The primary observational sequence, S0060, was designed to image the Sun one time in each of the four filters listed above and was given a medium priority for downlink. For highpriority operational use, S0059 imaged the Sun only in the red filter.…”

Mars Ozone: Mariner 9 Revisited

“…Scattering of radiation by dust is represented by the Henyey-Greenstein phase function, with an asymmetry parameter of 0.55 (Clancy and Lee, 1991). Typical cloud and fog opacities of 1.0 are taken from theory (Kulikov and Rykhletskii, 1983) and observations (Moroz, 1976;Pollack et al, 1977;Clancy and Lee, 1991), although large seasonal and latitudinal variations exist (Briggs and Leovy, 1974). A single-scattering albedo for cloud particles of 1.0 is taken from observations (Clancy and Lee, 1991).…”