The Diurnal Cycles of Dust and Water Vapor as Observed by EMIRS

Thermal infrared spectra taken by the Emirates Mars Infrared Spectrometer (EMIRS) are used to retrieve the diurnal variations of dust optical depth and water vapor column abundance in the Mars atmosphere. Using a retrieval algorithm optimized to determine the effective temperature of these constituents, we can accurately characterize their diurnal variations. Here, we present results from observations taken by EMIRS over more than 1.5 Mars Years. Along with the typical climatological patterns for water ice clouds and water vapor, the EMIRS observations show distinct periods of enhanced dust activity during both Mars Years 36 and 37, with four significant regional-scale dust storms observed but no planet-encircling events. We found small but systematic diurnal variations in both dust and water vapor. Dust optical depth typically increases from mid-morning through early afternoon, reaching peak values in late afternoon before decreasing through the night to minimum values before sunrise. The amplitude of this variation is 0.01–0.02 and is largest at seasons and latitudes where surface temperature is highest. The effective height of the dust layer is about one scale height during the aphelion season but increases to near two scale heights during the perihelion season and higher during dust storms. Water vapor shows modest diurnal variations of ∼10% with column abundance generally increasing during daytime hours, particularly in the morning.