THE NORTH POLAR HOOD IN EARLY AUTUMN ON MARS

We investigated the behavior of the north polar hood in early autumn on Mars, 1969 and 1986. The boundary of the north polar hood extended to a latitude of 30 degrees N similar to 40 degrees N in the early morning, and receded poleward to around 60 degrees N in the afternoon. Hence the difference between the boundaries in the morning and in the afternoon was more than 20 degrees in latitude. The optical depth of the north polar hood increased from the boundary toward the pole, and decreased to the evening limb. Most optical depths were more than unity in the early morning and less than 0.5 in the afternoon. The mean optical depths were 0.7 in 1969 and 0.6 in 1986 near local noon around a latitude of 60 degrees N. Bright spots appeared frequently in a lower latitude zone of the polar hood. They did not recede even in the afternoon. Their optical depths were higher than those in their surroundings. Most bright spots did not reappear on the following day, suggesting that they would die out during the night. However some bright spots survived a few days, moving east with a speed of about 700 km/day.