Retrievals of martian atmospheric opacities from MGS TES nighttime data

We describe a new retrieval algorithm that accounts for the presence of the systematic background radiance error in the infrared spectra collected by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES). The algorithm is used to retrieve dust and water ice cloud opacities from the nighttime TES nadir spectra. Nighttime dust opacities are systematically higher than the day opacities by ∼30%, which is attributed to uncertainties of the retrieval model. Nighttime water ice cloud opacities could be systematically underestimated by a factor of ∼2, depending on the ice particle size. Uncertainty of the ice particle sizes does not affect clouds spatial distributions. Independent of the ice particle sizes, the night clouds have higher opacity and are more extensive than the day clouds. Spatial and seasonal variability of nighttime ice clouds is investigated over a time span of 2.5 Mars years of TES observations. Seasonal evolution of the nighttime clouds is similar to the evolution of daytime clouds, with the maximum areal extent and highest opacities reached during the aphelion season (northern spring and summer) in the equatorial belt. A secondary increase in the ice cloud opacities is observed during perihelion season (southern spring and summer). During southern summer extensive clouds are found over and around the Tharsis volcanoes, Olympus Mons, western Valles Marineris, and between Syrtis Major and Elysium Planitia. This pattern of diurnal spatial variability is consistent with the strong influence of thermal tides on cloud formation. Perihelion season is characterized by the thinner nighttime clouds appearing south of the Tharsis and western Valles Marineris and north of the Hellas basin. The seasonal spatial patterns of the nighttime water ice clouds are highly repeatable year over year.