Seasonal recession of Mars' south polar cap as seen by Viking

The spring‐summer retreat of the south polar cap of Mars is portrayed in photomosaics obtained by Viking Orbiter 2 during 1977. Comparisons of these data to Mariner 9 photos and to the record of telescopic observations attest that the polar retreat viewed by Viking was significantly slower than those previously reported. A global dust storm which occurred at an unusually early season may have effected this retarded recession by introducing dust into the atmosphere of Mars which modified the polar energy balance through scattering of incident radiation. The composition of the south residual cap cannot be unambiguiously determined at this time; however, some data suggest that CO 2 or clathrate survived the entire summer viewed by Viking.