Martian dust storms: 1999 Mars Orbiter Camera observations

The Mars Orbiter Camera on board the Mars Global Surveyor from March 9, 1999 ( L s = 107°, where L s is the areocentric longitude of the Sun measured in degrees from Mars' northern spring equinox), to December 31, 1999 ( L s = 274°), has obtained 7.5 km pixel −1 daily global maps of the Martian 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 resulted in the detection of 783 dust storms, ranging in size from “local” (>10 2 km 2 ) to “regional” (>1.6×10 6 km 2 ). No global storms were observed. The difference in the numbers of dust storms occurring in the two hemispheres (north: 343; south: 440) was not statistically significant. Fifty percent 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° to L s = 223° were observed to move from the northern hemisphere into the southern hemisphere between 30°W and 50°W longitude. This cross‐equatorial transport of dust suggests the location of the low‐lying southward flowing branch of the Hadley circulation. 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 hood during northern 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 time frame. One region of exceptional activity not noted previously was the Arcadia‐Amazonis border. However, other regions, such as near Elysium Mons, Acidalia, Chryse, Hellas, Noachis, Argyre, Cimmeria, and Sirenum, again were active dust storm regions. Relative dust opacities were modeled for 117 red wavelength events and ranged from 0.54 “diffuse haze” to 2.13 “concentrated local events.” From the dust opacities we estimated the north polar sedimentation rate to be 6.0–10×10 −4 g cm −2 yr −1 , a factor of 40 smaller than previous estimates from the two 1977 global dust storms Pollack et al. , 1979, along with the global dust mass loading (2.6–4.0×10 −4 g cm −2 ) and cross‐equatorial dust mass loading (3.6×10 −4 g cm −2 ). The 1999 cross‐equatorial mass dust loading suggests that the southern hemisphere subtropical latitudes require at least 2–3 Martian years to replenish their dust sources before global storms can form.