This mosaic of Viking Orbiter images from September 1977 shows the appearance of the south polar cap in late summer, much reduced in size from its winter appearance. Although Mars is closest to the Sun during southern spring and summer, measurements taken at that time by the Viking Orbiters indicate that the local surface temperature never exceeded the 148 K frost point of CO2. This suggests that even at its minimum extent, the south polar cap retains at least a thin veneer of CO2 frost. However, it is believed that the dominant constituents of the residual cap are, as in the north, water ice and dust. For scale, the crater in the lower lefthand corner (named “South”) is approximately 100 kilometers in diameter, while the residual cap itself is 350–400 kilometers across.
Image processing by U.S. Geological Survey