MARTIAN LATE-NORTHERN-WINTER POLAR HOOD OPACITIES AND NON-VISIBILITY OF A SURFACE CAP - 1975 AND 1990 OBSERVATIONS

In the 1975 and 1990 apparitions of Mars, the north polar hoods were a lways seen bright on blue images. The boundaries of the polar hoods la y in mid latitudes of 30-degrees-N to 45-degrees-N, and the maximum br ightness of the polar hood appeared between 40-degrees-N and 55-degree s-N. The average value of opacity of the north polar hood was 0.4 in 1 975 and 0.5 in 1990. High opacities ranging from 0.7 to 1.4 were obser ved in a period from 23 October to 1 November 1990, abruptly. The pola r hood whose opacity is less than unity is not detected in red-filter photographs, and surface features are little obscured by the hood. The polar cap would be seen through the hood in red light, if the cap gro ws large enough to be observed from the Earth. The fact that the north polar cap was not observed in red in 1975 and 1990 suggests that the polar cap did not extend to 65-degrees-N in 1975 and to 55-degrees-N i n 1990.