Diurnal Variability in EMIRS Daytime Observations of Water Ice Clouds During Mars Aphelion‐Season. Issue 15 (11th August 2022) - Atwood, Samuel A.
Abstract: Diurnal analyses of water ice cloud optical depths retrieved from thermal infrared spectra by the Emirates Mars Infrared Spectrometer showed changing cloud abundance throughout the Martian day. Observations began with the start of the Emirates Mars Mission science phase near the beginning of aphelion‐season in Mars Year 36 and included the prominent aphelion cloud belt (ACB) and orographic clouds in the vicinity of volcanoes. A midday minimum with higher morning and afternoon optical depths was typical for the ACB, though with considerable spatial variability in this diurnal pattern. Clouds near volcanoes reached a minimum before local noon and tended to increase in abundance throughout the afternoon. Comparisons against the Laboratoire de Météorologie Dynamique global circulation model showed analogous spatial patterns in the diurnal signal, which suggested thermal tides and topographic effects to be the predominant drivers of ACB variability, while more localized circulations affected volcano clouds. Plain Language Summary: Observations from the Emirates Mars Infrared Spectrometer onboard the Emirates Mars Mission (EMM) spacecraft were used to measure the abundance of clouds in the Martian atmosphere and investigate how they changed throughout the day. Due to the unique nature of EMM's high orbit, the observations provided by EMIRS cover all times of day and provide more detailed information about how clouds change as compared to many previous missions. In these results we present information about this daytime cloud variability for different regions on Mars. A prominent region of clouds that is commonly observed near the equator during Mars' cold season—known as the aphelion cloud belt—was observed to reach a minimum near midday, with more clouds typically observed in both the morning and afternoon. Distinct differences were found in clouds observed near volcanoes, which tended to reach a minimum before local noon and increase throughout the afternoon. These results add detail to our understanding of cloud behavior and help us to validate computer models of the Martian atmosphere. Key Points: Infrared spectra from the Emirates Mars Infrared Spectrometer were used to obtain water ice cloud optical depths throughout the day The aphelion cloud belt had a midday minimum with higher optical depths in the morning and afternoon Orographic clouds near volcanoes were observed to increase throughout the afternoon