How well do we understand Mars great dust storms?

The most interesting feature of Mars' climate system is the two-way coupling between dust and the general circulation. Positive feedback sometimes causes intense regional dust storms to grow large enough to encircle the planet in the tropics and spread to higher latitudes. The Mariner 9 and Viking 1 and 2 spacecraft fortuitously collected data during four such great dust storms. Recently, general circulation models have provided new insights into the effects of dust on the general circulation, the transport of dust, and the initiation mechanisms of the great dust storms. Although these model results compare favorably with existing observations, planned spacecraft missions promise a welcome opportunity to check, improve, and extend the existing models. Dust storms interact with the water and carbon dioxide cycles and are sensitive to orbital parameters. Reconstructing the response of Mars' climate to orbital parameter variations will require clear understanding of the initiation and behavior of great dust storms.