Mapping Mariner 9 dust opacities

We present the results from a study of the spatial and temporal 9-mu m opacity variation of the Martian 1971 global dust storm. T. Z. Martin (1986, Icarus 66, 2-21) designed a procedure to calculate 9-mu m opacities from the Viking IR Thermal Mapper data set. We have convolved the Mariner 9 IRIS data with Viking IRTM filter functions in order to use Martin's approach. We find a mean opacity of 0.32 and a modal opacity of 0.21. The resulting opacities are mapped and binned by solar longitude in order to investigate the structure of the 1971 dust storm. There is a diminishing spatial variation in opacity as the storm decays. The highest opacities occur in the southern tropics while the lowest opacities consistently appear in the southern polar region and near the equator. Hellas is relatively clear at the peak of the storm but becomes dustier than the rest of the planet as the storm dissipates. This behavior may be typical of Hellas and Argyre during global dust storms. In contrast, the Tharsis plateau is relatively dusty early in the mission but clears quickly, We have also calculated storm decay constants for different regions. The storm has a decay constant of 67 days in the 20 degrees-30 degrees S zone, 42 days in the 55 degrees-65 degrees S zone, and 46 days on the Tharsis plateau. (C) 1997 Academic Press.