Water vapor near Venus cloud tops from VIRTIS-H/Venus express observations 2006–2011. (August 2015) - Cottini, V.
Abstract: This work aims to give a summary of the water vapor at the cloud top of Venus atmosphere using the complete set of observations made using high spectral resolution channel (-H) of Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), on board the ESA Venus Express orbiter, to measure the cloud top altitude and the water vapor abundance near this level. An initial analysis of these measurements byCottini et al. (2012) was limited to data in 140 orbits in the period 2007–2008. These observations were limited to the Northern hemisphere due to observational geometry in this early part of the mission. In the present paper, the analysis is extended to a larger dataset covering the years 2006–2011, significantly improving the latitudinal coverage. Altitude of the cloud tops, corresponding to unit optical depth at a wavelength of 2.5 µm, is equal to 69±1 km at low latitudes, and decreases toward the pole to 62–64 km. The water vapor abundance is equal to 3±1 ppm in low latitudes and it increases reaching a maximum of 5±2 ppm at 70–80° of latitude in both hemispheres, with a sharp drop in the polar regions. This can be explained by the specific dynamics of the atmosphere of Venus affecting the distribution of water vapor such as the transfer of water vapor in the Hadley cell and the dynamic in the polar vortex. The average height of the cloud tops and the H2 O near this level are symmetric with respect to the equator. As a function of local solar time, the water vapor shows no particular dependence, and the cloud tops exhibit just a weak maximum around noon. Over 5 years of observations the average values of the cloud top altitude and the water vapor were quite stable in low and middle latitudes, while in high latitudes both quantities in 2009–2011 years are systematically higher than in 2006–2008. Short period variations increasing with latitude are observed, from approximately less than ±1 km for cloud tops and ±1 ppm for water vapor in low latitudes to, respectively, ±2 km and ±2 ppm in high latitudes. As a rule there is no correlation between variations of the cloud top altitude, the water vapor content, and the UV brightness. However, numerous examples can be found when UV dark features, with a characteristic size of a few degrees of latitude (several hundred kilometers), coincide with regions of higher cloud tops. Highlights: Water vapor abundance at the cloud tops of Venus with high spectral resolution. Complete VIRTIS-H / Venus Express day-side dataset analyzed. Better coverage in latitude shows cloud top heights and H2 O symmetric around equator. No considerable dependence on local time is observed. Long term stability of water vapor at low latitudes with some variations over 60°.