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B. A. Cantor (Malin Space Science Systems and The University of Toledo, Department of Physics and Astronomy), P. B. James (The University of Toledo, Department of Physics and Astronomy), M. Caplinger (Malin Space Science Systems), M. J. Wolff (Space Science Institute and The University of Toledo, Department of Physics and Astronomy)
The Mars Orbiter Camera on board Mars Global Surveyor from March 9, 1999 (Ls = 107 \circ, the areocentric longitude of the Sun measured in degrees from Mars' northern spring equinox) to September 30, 2000 (Ls = 56\circ) has obtained daily global maps of the Martian surface at a resolution of 7.5 km/pixel in two wavelength bands: violet (400-450 nm) and red (575-625 nm). Visual inspection of these maps during the 1999-2000 dust storm season has resulted in the detection of over 1500 dust storms, ranging in size from ``local" (> 102 km2) to ``regional" (> 106 km2) storms. No global storms were observed. Fifty percent of the regional size dust storms formed as a result of the ``merger" of two or more smaller local storms. The spatial and temporal distribution of these dust events as well as the dust opacity are discussed.
Observations show that dust storms occur in several regions on Mars: at the two polar cap edges, at the base of high elevation regions in the northern hemisphere, near the polar hoods during the fall, and at mid-latitudes in both hemispheres. Specific regions such as Solis Planum and Hesperia, which were regions of significant dust activity in the past, showed almost no dust storm activity in 1999, suggesting that regional dust sources are variable over a 20 year period.
Relative dust opacities for over 120 dust storms observed with the MOC red filter were modeled by adopting the discrete-ordinate, multiple scattering radiative transfer code (DISORT). Dust opacities ranged from 0.54 ``diffuse haze" to 2.13 ``concentrated local events". From the dust opacities, we estimated the 1999 cross-equatorial dust mass loading (3.6x10-4 g/cm2). This dust mass loading suggests that the southern hemisphere subtropical latitudes require at least two to three Martian years to replenish their dust sources before global storms can form.