OUT ON A LIMB - MARTIAN ATMOSPHERIC DUST OPACITY DURING THE PAST 100 YEARS

Measurements of the radius of Mars obtained using Earth-based telescop ic measurements between 1879 and 1958 determined the altitude in the a tmosphere at which the line-of-sight opacity was unity. These data can be used to estimate the dust content of the martian atmosphere during that time period. Comparison with the distribution of brightness seen in images of the limb from the Mariner 9 and Viking spacecraft sugges ts that the dust content prior to 1956 was substantially less than dur ing the 1970s. In addition, the dust content in the polar regions was substantially less than in the equatorial regions. These data suggest that there is significant variability of the dust cycle on decade- or century-long time scales. The atmospheric behavior seen by Viking appe ars to represent an extreme behavior rather than one typical or repres entative of the present epoch. (C) 1995 Academic Press, Inc.