The "foggy" limb of Mars

The first Soviet Orbiters, Mars-2 and Mars-3, arrived on November 27 and December 2, 1971. They carried the 52mm "Vega" and 350mm "Zulfar" phototelevision cameras (later used on Mars-4 and 5), carrying enough film for 480 photos per camera. These cameras were designed by A.S. Selivanov and his team at the Institute of Space Device Engineering.
The original plan was to spend the first 40 days performing photographic surveys; however, Mars was experiencing a severe planet-wide dust storm when the Soviet probes (and the American Mariner-9) arrived.
This photo left shows the atmospheric limb during the dust storm, from a distance of about 150.000 Km (the orbit of Mars-3 was highly elongated). Mars-2 did not return much data because of problems with its telemetry systems.