Seasonal recession of Mars' South polar cap in 1986

Photographs of Mars obtained during the 1986 opposition of the planet have been used to derive the regression curve for the south polar cap between L s = 190° and L s = 255°. The 1986 regression appears to have been unexceptional until after L s = 230°, when it becomes retarded relative to the normal established by the 1971 and 1977 regressions. A study of the data as a function of the filter used suggests that circumpolar clouds were present in early spring, unlike the case in the 1977 recession.