Martian dust storms: 1999 Mars Orbiter Camera observations

The Mars Orbiter Camera on board the Mars Global Surveyor from March 9, 199 9 (L-S=107 degrees, where L-S is the areocentric longitude of the Sun measu red in degrees from Mars' northern spring equinox), to December 31, 1999 (L -S=274 degrees), has obtained 7.5 km pixel(-1) daily global maps of the Mar tian surface in two wavelength bands: blue (400-450 nm) and red (575-625 nm ). Visual inspection of these maps during the 1999 dust storm season has re sulted in the detection of 783 dust storms, ranging in size from "local" (> 10(2) km(2)) to "regional" (>1.6x10(6) km(2)). No global storms were obser ved. The difference in the numbers of dust storms occurring in the two hemi spheres (north: 343; south: 440) was not statistically significant. Fifty p ercent of the regional size dust storms formed as a result of the,"merger" of two or more smaller local storms. The regional storms from L-S = 207 deg rees to L-S = 223 degrees were observed to move from the northern hemispher e into the southern hemisphere between 30 degreesW and 50 degreesW longitud e. This cross-equatorial transport of dust suggests the location of the low -lying southward flowing branch of the Hadley circulation. Observations sho w that dust storms occur in several regions on Mars: at the two polar cap e dges, at the base of high elevation regions in the northern hemisphere, nea r the polar hood during northern fall, and at mid-latitudes in both hemisph eres. Specific regions such as Solis Planum and Hesperia, which were region s of significant dust activity in the past, showed almost no dust storm act ivity in 1999, suggesting that regional dust sources are variable over a 20 -year time frame. One region of exceptional activity not noted previously w as the Arcadia-Amazonis border. However, other regions, such as near Elysiu m Mons, Acidalia, Chryse, Hellas, Noachis, Argyre, Cimmeria, and Sirenum, a gain were active dust storm regions. Relative dust opacities were modeled f or 117 red wavelength events and ranged from 0.54 "diffuse haze" to 2.13 "c oncentrated local events." From the dust opacities we estimated the north p olar sedimentation rate to be 6.0-10x10(-4) g cm(-2) yr(-1)., a factor of 4 0 smaller than previous estimates from the two 1977 global dust storms [Pol lack et al., 1979], along with the global dust mass loading (2.6-4.0x10(-4) g cm(-2)) and cross-equatorial dust mass loading (3.6x10(-4) g cm(-2)). Th e 1999 cross-equatorial mass dust loading suggests that the southern hemisp here subtropical latitudes require at least 2-3 Martian years to replenish their dust sources before global storms can form.