Mars' south polar hood as observed by the Mars Climate Sounder

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Abstract
We have used observations from the Mars Climate Sounder to investigate south polar hood water ice clouds (at 12 μm), including the first systematic examination of the vertical (5 km resolution) and nighttime structure. We find that the structure and evolution of the polar hood is controlled more strongly by atmospheric temperature variations than by intrinsic fluctuations in water vapor abundance. 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°S and 75°S with a visible column optical depth between 0.075 and 0.15. The cloud belt then evaporates as temperatures warm. During phase 2, the cloud belt reappears due to an increase in water vapor as a partial band of low‐opacity clouds south of the Tharsis region and eventually becomes continuous in longitude, with a visible column opacity between 0.125 and 0.25. As the southern spring equinox approaches, the cloud belt shifts southward, following the seasonal cap edge. From LS = 140° to LS = 200°, the daytime belt lies about 15° farther south than the nighttime belt, due to tidally driven diurnal temperature differences. 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 nighttime and daytime because of thermal tidal control of the condensation altitudes. Overall, the southern polar hood is observed to rapidly form and dissipate as the temperature crosses the saturation point of water vapor.
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