Spacecraft Observations of the Martian Atmosphere*

Recent spacecraft observations of the Martian atmosphere show that the current climate has a distinct and generally repeatable pattern of seasonal and spatial variations. The half of the year that Mars is closest to the Sun is characterized by warm, dusty conditions, whereas the opposite season is relatively cool, free of dust, and dominated by a low-latitude band of water ice clouds. Water vapor is most abundant at high latitudes in the summertime of both hemispheres, whereas a polar hood of clouds forms over the winter polar region. The greatest year-to-year variations are caused by intermittent large dust storms, which also affect atmospheric temperature and circulation patterns on a global scale.