Mechanisms for Mars Dust Storms

Abstract

Characteristics of the Mars global dust storm are reviewed. At the Mariner 9 encounter, the dust consisted of highly absorbing particles distributed rather uniformly up to great height (?50 km). These observations together with temperature distributions inferred from the Mariner 9 IRIS by Hanel and his collaborators are used to estimate global wind systems during the dust storm. The global distribution and direction of light surface streaks indicate that the axially symmetric circulation was a dominant part of flow during the dust storm. An energy balance argument is used to estimate the intensity of the equatorial part of such a wind system. Surface winds driven by the diurnal and semidiurnal components of solar heating are also estimated. The axially symmetric winds may become strong enough to raise dust over wide areas of Mars' tropics under unusual conditions: the incoming solar radiation must be near its seasonal maximum, the static stability must be low, and the atmosphere must be able to absorb and re-emit a sizeable fraction of the incoming radiation. Emission is aided by the formation of H20 ice clouds in the winter northern polar region. Absorption is enhanced by the presence of a small dust opacity in the atmosphere prior to the onset of the global dust storm. Strong winds around the periphery of the retreating south polar cap would be driven by the temperature gradient at the cap edge and by the mass outflow due to subliming C02. These polar winds could generate local dust storms, raising the general level of dustiness, and providing the conditions necessary for onset of a global dust storm. Observational evidence for this sequence of events is discussed. The proposed model is sensitive to the precise phase relationship between Mars' perihelion and southern summer solstice, and variations in this phasing may have caused a strongly episodic behavior of dust storms and of a number of related planetary processes.