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

•H2O mixing ratio and the cloud top altitudes were measured by SPICAV IR on VEx over 8.5 years.•The average H2O mixing ratio equals 5–7ppm at effective altitudes of 60–62km.•Two maxima in the latitudinal distribution of H2O were observed: near equator and near the pole.•A asymmetry of H2O longitudinal distribution has been observed in low latitudes.•No prominent long-term nor local time variations of H2O and the cloud tops were detected. SPICAV VIS-IR spectrometer on-board the Venus Express mission measured the H2O abundance above Venus’ clouds in the 1.38µm band, and provided an estimation of the cloud top altitude based on CO2 bands in the range of 1.4–1.6µm. 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 1.48µm, varies from 68 to 73km at latitudes from 40ºS to 40ºN with an average of 70.2±0.8km assuming the aerosol scale height of 4km. In high northern latitudes, the cloud top decreases to 62–68km. The altitude of formation of water lines ranges from 59 to 66km. The H2O mixing ratio at low latitudes (20ºS-20ºN) is equal to 6.1±1.2ppm with variations from 4 to 11ppm and the effective altitude of 61.9±0.5km. Between 30º and 50º of latitude in both hemispheres, a local minimum was observed with a value of 5.4±1ppm corresponding to the effective altitude of 62.1±0.6km and variations from 3 to 8ppm. At high latitudes in both hemispheres, the water content varies from 4 to 12ppm with an average of 7.2±1.4ppm which corresponds to 60.6±0.5km. 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 t