Venus Mesospheric Water Vapour From VIRTIS-H VEX Dayside Measurements

The Venus-Express mission works in orbit around Venus since April 11th 2006. VIRTIS-H is a high resolution (1-3 nm) IR channel of the VIRTIS mapping spectrometer. Venus nadir dayside spectra measured with VIRTIS-H have been used to derive simultaneously the altitude of the cloud tops and water abundance in the mesosphere. A line-by-line multiple scattering code has been used to fit the CO2 bands at 2.48 micron and H2O bands at 2.56 micron. Clouds have been simulated with "mode 2" particles of 75 percent H2SO4 and exponential vertical profile in the mesosphere with the scale height of 4 km. Highest quality data has been selected from 8 orbits with local times from 10 to 15 h, and a special attention has been given to the regions near the local noon. We obtained an average cloud top altitude of 75 +/-1 km for low latitudes regions, equator ward of 30 degrees. Results for higher latitude are under examination. Water vapour abundance in low latitudes regions is found to be about 1 ppm at 75 km, with weak variations in the selected orbits and up to 2 -3 ppm in the others, besides with no indication of enhancement observed in 80s from Pioneer Venus observations. Our measurements of cloud top altitude and H2O abundance agree well with other VEX observations: VIRTIS-M and SPICAV, respectively.