Mars’ South Polar Hood as Observed by the Mars Climate Sounder

We have used observations from the Mars Climate Sounder (MCS) to investigate the south polar hood water ice clouds, including the first systematic examination of the vertical and nighttime structure. The structure and evolution of the polar hood clouds is dominated by the atmospheric thermal structure, with variations in water vapor abundance playing a secondary role. The clouds form as a belt during Ls=10°-70° (phase 1) and Ls=100°-200° (phase 2). During phase 1, the cloud belt extends over a wide latitude range, between 30° and 75° S with a visible optical depth between 0.075 and 0.15. By the end of this period the AM clouds have dissipated while in the PM there is still a distinct belt with lower opacity clouds around 60° S. During phase 2, the clouds first form as a partial band of low opacity clouds south of the Tharsis region, and eventually become continuous in longitude, first in the PM at about Ls=130° and then in the AM around Ls=145°, with a visible optical depth between 0.125 and 0.25. As the season advances, the cloud belt shifts southward, following the seasonal cap edge. From about Ls=140°-200°, the PM belt lies about 15° further south than the AM belt. The vertical structure of the cloud belt is consistent within and between the two seasonal phases and is characterized by a thick lower cloud deck and an upper layer whose altitude shifts between the AM and PM because of thermal tidal control of the condensation altitudes.