Viking photographs a Martian dust storm. In this picture from Viking Orbiter 2, a turbulent, bright dust cloud (arrow) more than 300 kilometers (186 miles) across can be seen inside the great Argyre Basin. It is apparently moving eastward under the influence of strong winds that also create condensate lee-wave clouds to the west of the basin. This is the first color picture of a dust storm taken from a spacecraft orbiting the planet. Large depressions like Argyre and Hellas seem to be favored locations for the formation of dust storms. Later this year, when Mars comes closest to the Sun, a local dust storm like this may spread violently and cover most of the planet. A global dust storm delayed the start of Mariner 9's mapping mission in 1971. Although Viking scientists do not welcome the likely interruption of the mapping from orbit, the possibility of studying a global dust storm from its beginning is recognized as an unusually interesting phenomenon. The great Argyre Basin in the southern hemisphere of Mars is one of several enormous depressions created by the impact of large asteroids early in the planet's history. Previous Viking images and infrared data have shone that during winter the floor of Argyre is covered with carbon dioxide frost -- the edge of the south polar ice cap. In this view of the region, at the end of southern winter, the bright polar cap covers only the southern half of Argyre -- the cap has begun to retreat. In a few places among the mountains surrounding the basin the ice can be seen as white. Elsewhere the polar cap has a golden hue. Surface dust, apparently suspended in the atmosphere, filters the light from the cap and scatters light from the sun
