Solar occultation measurements of the Martian atmosphere on the Phobos spacecraft: Water vapor profile, aerosol parameters, and other results

The Auguste experiment onboard the Phobos spacecraft was devoted to solar occultation spectroscopy of the Martian atmosphere in the ultraviolet through infrared wavelength region. Despite the short duration of the space mission and problems associated largely with a fault in the solar pointing system, data have been obtained on the chemical composition and aerosol content in the atmosphere of Mars at sunset early in the summer at equatorial latitudes (in the northern hemisphere). This paper presents a somewhat detailed review of the experiment performed, the data obtained, and their interpretation, and compares these data with new results. Ozone traces were detected at attitudes of 40-60 km, and, in one case, an ozone profile was obtained. Nine profiles of water vapor content at altitudes between 12 and 50 km were obtained from absorption data in the 1.87-mum band. At altitudes of 23-25 km, the mean H2O concentration profile falls steeply to the value of similar to3 ppm, but at lower altitudes the relative H2O content is approximately constant (similar to130 ppm). The overall content of water vapor is estimated as 8.3(-1.5)(+2.5) mum of settled water. The temperature profile for the saturated atmosphere yields a cooling rate of 2 +/- 1 K/km at attitudes from 25 to 35 km. The atmospheric extinction profiles were measured at altitudes from 10 to 50 km at the wavelengths 1.9 and 3.7 mum. The atmosphere is transparent up to 25-33 km; below this level radiation is attenuated by dust; it is also possible that a layer of water ice clouds is present at altitudes of 20-25 km. High-altitude transparent (tau = 0.03) clouds consisting supposedly of water ice were observed in 5 of 38 cases at altitudes z approximate to 50 kin. The optical depth To of the atmosphere was estimated to be 0.2 +/- 0.1, and constraints on the form of the size distribution of dust particles were established. Spectral features in the 3.7 mum range have been previously attributed to formaldehyde; its content is substantially higher than the limits deduced from new ground-based observations. The spectrum in the 3.7 mum range is discussed and other unsettled problems are pointed out.