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. Large depressions like Argyre and Hellas seem to be favored locations for formation of dust storms. Later this year, when Mars comes closest to the Sun, a local dust storm like this one 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 their 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 shown that during winter the floor of Argyre is covered with carbon dioxide frost -- the south polar ice cap.
