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

We have used observations from the Mars Climate Sounder (MCS) to investigate the north polar hood (NPH) water ice clouds, including the first systematic examination of the vertical and nighttime structure. We show that the NPH clouds are present between L-S = 150 degrees (early autumn) and 30 degrees (late spring) and that the clouds always extend to the pole. The daytime (1500 LMST) and nighttime (0300 LMST) clouds both have one layer that extends in altitude from 10 to 40 km above the surface, and the layer falls from its peak with a constant mixing ratio. We find that the cloud optical depth is controlled by the atmospheric thermal structure. The nighttime optical depth values are often higher than the daytime, sometimes due to tidally driven diurnal temperature differences and other times (i.e., L-S = 240 degrees-330 degrees) a result of low temperatures associated with the polar vortex at night. We conclude that polar hood clouds are primarily controlled by the temperature structure and form at the water condensation level.