Cloud structure in Venus middle-to-lower atmosphere as inferred from VEX/VIRTIS 1.74 mum data

International audienceWe have analyzed 1.74 mum nightside emission of Venus recorded using Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) onboard European Space Agency's (ESA) Venus Express (Orbit 344, 30-31 March 2007). Attention was paid to how infrared radiance, intense at the center of the 1.74 mum ``window'', dims at an off-center wavelength (1.71 mum). Cloud models are required to simultaneously reproduce the emission intensity at 1.74 mum and the ratio of intensities (I 1.71mum/I 1.74mum). Our best-fit model (5 km vertical resolution) has located the main cloud opacity in 40-45 km altitude, lower than previous studies. This may be due to the use of CO2 line parameters from a relatively new source (Carbon Dioxide Spectroscopy Databank) which may also be responsible for weaker continuum absorption, 5.6 × 10-9 cm-1 amagat-2. The data are reproduced well by models of which total aerosol optical thickness is 30-50 plus subcloud haze at 30-40 km altitude. We have mapped the subcloud haze opacity (approximately 0-4) and found that the opacity basically anticorrelates with the 1.74 mum intensity. There are regions of ``positive'' correlation which may imply enhanced production of aerosols due to penetration of more sunlight in less cloudier regions. Venus Express, now with a capability of sensing ``from the top to the bottom'' of Venus cloud system, will greatly enhance our knowledge about the current status of Venus atmosphere