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 degrees and L-s = 330 degrees) 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 similar to 90-110 nm assuming a monodisperse distribution. The altitude profile of extinction indicates that the cloud layers are confined horizontally to sizes less than similar to 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.