Variations of water vapor and cloud top altitude in the Venus' mesosphere from SPICAV/VEx observations

SPICAV VIS-IR spectrometer on-board the Venus Express mission measured the H2O abundance above Venus' clouds in the 1.38 mu m band, and provided an estimation of the cloud top altitude based on CO2 bands in the range of 1.4-1.6 mu m. The H2O content and the cloud top altitude have been retrieved for the complete Venus Express dataset from 2006 to 2014 taking into account multiple scattering in the cloudy atmosphere. The cloud top altitude, corresponding to unit nadir aerosol optical depth at 1.48 mu m, varies from 68 to 73 km at latitudes from 40 degrees S to 40 degrees N with an average of 70.2 +/- 0.8 km assuming the aerosol scale height of 4 km. In high northern latitudes, the cloud top decreases to 62-68 km. The altitude of formation of water lines ranges from 59 to 66 km. The H2O mixing ratio at low latitudes (20 degrees S-20 degrees N) is equal to 6.1 +/- 1.2 ppm with variations from 4 to II ppm and the effective altitude of 61.9 +/- 0.5 km. Between 30 degrees and 50 degrees of latitude in both hemispheres, a local minimum was observed with a value of 5.4 +/- 1 ppm corresponding to the effective altitude of 62.1 +/- 0.6 km and variations from 3 to 8 ppm. At high latitudes in both hemispheres, the water content varies from 4 to 12 ppm with an average of 7.2 +/- 1.4 ppm which corresponds to 60.6 +/- 0.5 km. Observed variations of water vapor within a factor of 2-3 on the short timescale appreciably exceed individual measurement errors and could be explained as a real variation of the mixing ratio or/and possible variations of the cloud opacity within the clouds. The maximum of water at lower latitudes supports a possible convection and injection of water from lower atmospheric layers. The vertical gradient of water vapor inside the clouds explains well the increase of water near the poles correlating with the decrease of the cloud top altitude and the H2O effective altitude. On the contrary, the depletion of water in middle latitudes does not correlate with the H2O effective altitude and cannot be completely explained by the vertical gradient of water vapor within the clouds. Retrieved H2O mixing ratio is higher than those obtained in 2.56 mu m from VIRTIS-H data (Cottini et al., [2015] Planet Space Sci., 113, 219-225) at altitudes of 68-70 km which is well consistent with the lower altitudes of water mixing ratio from the 1.38 mu m band. Observations for different solar and emission angles allowed to constrain also the average vertical distribution of H2O mixing ratio in the clouds with 2 ppm at 66 km and 7-7.5 ppm at 59-61 km. The water vapor latitudinal-longitudinal distribution does not show any direct correlation with the cloud tops. Yet a strong asymmetry of H2O longitudinal distribution has been observed with a maximum of 7-7.5 ppm from 120 degrees to 30 degrees of longitude and shifted to the southern hemisphere (20 degrees S-10 degrees N). To the east, the minimum is observed with values not in excess of 6 ppm and over a wide range of longitudes from 30 degrees to 160 degrees. Bertaux et al. (2015) announced a correlation between the zonal wind pattern in the equatorial region and underlying topography of Aphrodite Terra as the result of stationary gravity waves produced at the ground level near the mountains. The water minimum corresponds to the Aphrodite Terra highlands and can be also associated with the influence of Venus topography. No prominent long-term on the time scale of 8. 5 years nor local time variations of water vapor and the cloud top altitude were detected. (C) 2016 Elsevier Inc. All rights reserved.