Behavior of the Martian north polar cap, 1975–1978

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Abstract
In the photographic observations of Mars at the Kwasan Observatory and at the Hida Observatory during the 1977–1978 apparition, the north polar cap was first observed at Ls = 14°, and the latitude of the edge of the cap was about 65°N. This edge of the cap changed little until Mars reached Ls = 50°, and then the cap started to shrink. The edge was observed near 74°N at Ls = 70°. The relatively constant dimension of the north polar cap observed before Ls = 50° appears to be a regular phenomenon in the behavior of the north polar cap. The photographs obtained at the Hida Observatory during the 1975–1976 apparition were examined to study the nature of the north polar cap near Ls = 0°. Red filter photographs taken in the period from Ls = 340°–0° revealed dark surface features of the northern high latitudes, when blue filter photographs show the extensive polar brightening (i.e., the polar hood). The latitude of the northernmost dark feature observed was about 65°N. A wide spread north polar cap, which is predicted by most of the existing models, was not observed in our observations in 1975–1976.