Systematic Mariner 9 monitoring of the space and time distribution of Martian bright and dark markings, the streaks and splotches, indicates a range of global correlations. The time variable classical dark markings owe their configurations and variability to their constituent streaks and splotches, produced by windblown dust. Streaks and splotches are consistent wind direction indicators. Correlation of global streak patterns with general circulation models shows that velocities ∼50 to 90 m/sec above the boundary layer are necessary to initiate grain motion on the surface and to produce streaks and splotches. Detailed examples of changes in Syrtis Major, Lunae Palus, and Promethei Sinus are generally consistent with removal of bright sand and dust and uncovering of darker underlying material as the active agent in such changes, although dark mobile material probably also exists on Mars. The generation of streaks and the progressive albedo changes observed require only threshold velocities of about 2 m/sec for about 1 day at the grain surface. We propose that the dark collar observed following the north polar cap in its retreat is produced by the scouring of bright overlying dust from the polar peripheral ground by winds driven by the temperature differences between frosted and unfrosted terrain. The stability of bright streaks and the variability of dark streaks and splotches, as well as their contrast, can be the result of size differences of the constituent particles.
Following the accretion of solids and gases in the solar nebula, the giant planets contracted to their present sizes over the age of the solar system. It is presently hypothesized that this contraction was rapid, but not hydrodynamic; at a later stage, a nebular disk out of which the regular satellites formed may have been spun out of the outer envelope of the contracting giant planets due to a combination of total angular momentum conservation and the outward transfer of specific angular momentum in the envelope. If these hypotheses are true, the composition of the irregular satellites directly reflects the composition of planetesimals from which the giant planets formed, while the composition of the regular satellites is indicative of the composition of the less volatile components of the outer envelopes of the giant planets.
Background information is provided on the nature and importance of the El Chichon volcanic cloud and an introduction is given to the papers of this special issue.
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.
