The Full Diurnal Cycle of Mars Water-Ice Cloud Optical Depth in EMIRS Observations

Improvements to analyses of Martian thermal infrared spectra observed by the Emirates Mars Infrared Spectrometer (EMIRS) allow for retrieval of water-ice cloud optical depth, τ_ice, across the full diurnal cycle at Mars. Using observations spanning nearly two Martian years, we find a persistent pattern of higher nighttime cloud abundance compared to daytime values, with regular peaks in τ_ice occurring in both morning and evening hours. During the colder aphelion season when the low-latitude aphelion cloud belt forms, zonal mean optical depths ranged from maxima of ∼0.5 during the morning peak to minima of ∼0.15 near midday. Averaged across the full data set, nighttime τ_ice values were approximately twice the optical depth at the midday minimum. The morning peak in cloud abundance tended to occur near a solar incidence angle of 75° across a range of latitudes and seasons. These diurnal patterns were generally consistent with modeled influences of atmospheric thermal tides on cloud formation conditions. Spatial differences in the distribution of clouds were noted across the diurnal cycle. Volcano clouds formed near midday and increased in optical depth through the afternoon and into the evening. At night, late evening clouds were observed more broadly across low-latitude regions, while morning clouds were more concentrated over the Tharsis region. The broad spatial and temporal coverage of these results enables new model comparisons against observations of τ_ice across the full diurnal cycle, and advances our understanding of water-ice cloud development and its role in the Mars climate system.