Detection of Mesospheric CO₂ Ice Clouds on Mars in Southern Summer

This paper reports the first detections of two high-altitude nighttime CO₂ clouds on Mars during southern summer (L_s=264° and L_s=330°) with stellar occultation measurements by the Imaging Ultraviolet Spectrograph on board the Mars Atmosphere and Volatile Evolution spacecraft. Interpretation of the transmission spectra with Mie theory indicates particle radii of ∼90–110 nm assuming a monodisperse distribution. The altitude profile of extinction indicates that the cloud layers are confined horizontally to sizes less than ∼500–700 km. Examination of the CO₂ density and temperature profiles reveals strong wave-like perturbations. Supersaturated temperatures occur at the maximum negative extent of these wave-like perturbations, which are organized in longitude with a dominant m=3 zonal harmonic. This suggests that tides are important in the formation of CO₂ clouds.