Other
Scientific paper
Nov 1967
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1967saosr.255.....s&link_type=abstract
SAO Special Report #255 (1967)
Other
6
Scientific paper
A model of seasonal changes on Mars and related phenomena is developed with dust, winds, and elevation differences as the principal ingredients. A comparison of the velocities required for dust mobility on Mars with those predicted by theoretical meteorology, as well as observations of yellow clouds on Mars, indicates that grains on the Martian surface are moved by saltation, suspension, and creep. The velocity above the stress boundary layer is considered proportional to p-n where p is the atmospheric density. The equation of continuity implies that usually n = 1. If n > 2/3, the actual wind velocities will increase with elevation faster than the wind velocities needed for dust transport, and the frequency of dust storms in highlands, νh, will exceed that in lowlands, νl. If, in addition, the mean velocity is less than the threshold velocity for mobility, so that particle motion depends on the tail of the velocity distribution function, νh/νl >>1. The exchange of particles between highlands and lowlands should lead to a concentration steady state. For n > 2/3, Saltating particles will be concentrated in highlands, suspendible particles in lowlands, as the photometric and polarimetric observations imply. In this case, dark areas will be systematically highlands, bright areas lowlands. There will also be a tendency for the larger and darker particles to concentrate on the windward slopes of elevations. Mars exhibits very steep meridional temperature gradients in winter and very shallow such gradients in summer. This leads to significant changes in the zonal wind velocities and therefore in the mean particle size of the dark areas. For all reasonable velocity distribution functions, these particle-size changes are just the amounts implied by photometric and polarimetric observations of the wave of darkening. Springtime brightening of Martian bright areas, anomalous darkening of dark areas caused by the passage of dust clouds, and the existence and numerical value of a maximum darkening of the dark areas are all correctly predicted on this model. A number of other statistical regularities of the seasonal changes, as well as the rapid regeneration of dark areas after coverage by dust, the secular changes, and the seasonal changes of the canals all have a straightfoward explanation in the context of the windblown dust model.
Pollack James B.
Sagan Carl
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