ONSET

It seems likely that the origin of great dust storms on Mars is tied to interaction between suspended dust and wind which produces a positive feedback, allowing local and regional storms to grow to planet-encircling proportions. While it is important to keep in mind that much of our historical data is subject to an observational bias in which telescopic coverage varies with the apparent size of Mars, more recent and less seasonally biased data from orbiting spacecraft supports earlier conclusions that the great storms are seasonal in nature, all such storms having occurred within 90° in areocentric longitude of perihelion (corresponding to southern hemisphere spring and summer).

In general, Martian dust storms are concentrated latitudinally in zones equatorward of the seasonal polar caps and longitudinally in several distinct regions which have been observed to vary from one cycle to the next. Local and regional dust storms observed by the Mars Global Surveyor in 1999 (Cantor et al, 2001) provide evidence of this interannual variability, suggesting a 3-4 Mars year cycle of dust transfer from north to south by regional storms over 2-3 years, then south to north by global storms in the following season.

Processes which initially raise dust may include localized dust devils and interaction between regional winds (due to slope and horizontal temperature variations) and the general circulation patterns of the planet. Mars' extreme north-south topographic disparity plays a role in controlling the nature and location of these winds.

EVOLUTION

In comparison to Mars, Earth's dust storms are extremely localized and thus it is understandable that early observers sought alternative explanations of seasonal albedo changes. Kuiper's realization that this phenomenon was caused by the redistribution of dust was irrefutably confirmed by Mariner 9 and subsequent missions, which showed the extreme size, duration, and opacity of Martian dust storms. Orbital and telescopic data have shown that heating of atmospheric dust greatly alters the thermal structure and circulation patterns in the atmosphere (Zurek, 1992).

Studies of dust properties help provide bounds on composition, particle size distributions, and sedimentation rates. These data are a critical element of general circulation models for Mars which help us understand the processes driving the atmospheric behavior of the planet. State-of-the-art GCMs indicate that the primary mechanism responsible for transportation of dust to high altitude and across the equator is a Hadley circulation, which generally runs from the southern subtropics to the northern mid-latitudes on Mars. Traveling and stationary eddies then distribute the atmospheric dust further poleward (Murphy et al, 1995).

CESSATION

The least understood element of the dust storm cycle is their demise. High dust opacities in the atmosphere limit observation from orbit of the lower atmosphere, and ground coverage by the landers has been spatially insufficient to fully characterize cessation mechanisms. The theoretical negative feedback process in which atmospheric dust modifies the vertical temperature gradient, thereby decreasing surface winds and shutting off the dust supply, has yet to be consistently produced in GCMs.

After several months or years in the atmosphere, the dust eventually settles to the surface all over the planet, sometimes modifying the local surface brightness and thermal properties. By measuring surface albedo and thermal inertia, one can infer areas of net removal and deposition of dust subsequent to storm activity. In addition to the interannual variability discussed above, the longer term effect of planetary orbital evolution may cause a reversal of the cross-equatorial seasonal cycle and is likely to be important in controlling the development of the polar laminae.

2001 GREAT DUST STORMS

The recent global event documented by the Hubble Space Telescope and Mars Global Surveyor instruments gives us many new insights into dust storm onset, evolution, and cessation processes. Details of the characteristics similar to previously observed storms, including onset in the southern subtropical zone, regional storm coalescence, eastward zonal expansion of dust, and the temperature measurements should help refine existing GCMs for Mars. More intriguing are the unexpected early longitudinal expansion and the generation of several regional storms thought to have acted as multiple dust source areas. Analysis of these phenomena could lead to major revisions of both theory and modeling of the Martian atmosphere and its relationship with surface materials.

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Acknowledgments

The general layout of this web site was modeled after one by Shawn Brooks, which describes Titan's atmosphere . I'd also like to thank Dimitri Veras for a critical review of the layout and content. Finally, I'd like to acknowledge all those who provide the fantastic images and web sites which were the source of images and links available here.