Vertical Distribution of Water in the Near-Equatorial Troposphere of Mars: Water Vapor and Clouds

Abstract

Solar occultation spectrometry of the martian atmosphere was performed during the Phobos mission with the Auguste experiment in the UV, visible, and NIR spectral ranges. The entire domain of data in the range of 1.9 μm has been reanalyzed to retrieve all available information about the water vapor vertical profile. Accurate consideration of the instrumental corrections and a probabilistic concept of the inverse problem provide estimated water vapor contents at the level of 135 ppm with an accuracy of a factor of 2 at 10–25 km. Explicit detection of water above 30 km is proven statistically at the level of 3 ± 2 ppm with a variety of occultations, giving an upper limit of 10 ppm. The vertical structure of the water profiles demonstrates a sharp decrease above 23–25 km, whereas the lower portion has an approximately constant mixing ratio. The assumption that the latter is constant down to the surface results in a total column abundance of 8.3 +2.5 −1.5pr. μm. The temperature profile derived from saturation conditions reveals a gradient of 2 ± 1 km −1at 25–35 km. The vertical distribution of water vapor accounts for the previous study of continuous absorption at 1.9 and 3.7 μm that resulted in vertical profiles and size distributions of dust particles. The upper boundary of the confined part of the water vapor profile coincides with the altitude of considerable enhancement of aerosol extinction. This peculiarity is interpreted as a low level (20–25 km) of clouds formed by condensation of water vapor interacting with dust particles. Microphysical parameters of clouds are estimated with the adopted value of the eddy mixing coefficient, and constraints on size distribution are deduced from the modeling of dust dynamics.