Multivariate analysis of Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) Venus Express nightside and limb observations

Spectral signatures measured by Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the nightside and at the limb of Venus are analyzed with Independent Component Analysis. A methodology has been set up to minimize instrumental effects and to interpret the results on the basis of studies of the most common situations in the data set. The main spectral components commonly retrieved on the nightside include the bulk signal modulated by atmospheric opacity variations, photometric variations in the long-wavelength atmospheric windows, a branching parameter describing particle size variations, and O-2 emission at 1.26 and 1.58 mu m. Faint atmospheric windows are detected at 1.51, 1.55, 1.78, and 1.82 mu m for the first time. The polar vortex structure is outlined, with two main circular areas made of many concentric rings with alternating particle sizes. Discrete clouds about 100 km across are observed in low opacity conditions. High-altitude, warm clouds are tentatively observed from the polar vortex down to 55 degrees S. At the limb, the two signatures of CO2 nonlocal thermodynamic equilibrium emission are directly mapped, and the thermal structure of the cloud layers and upper atmosphere is apparent. Surface emission is detected with a spatial resolution limited by atmosphere blurring, reaching similar to 35 km in exceptional conditions. Horizontal offsets indicate that the radiation propagates mostly vertically, consistent with the large optical depth and vertical extent of the cloud layer. Intense scattering is suspected to take place at the bottom of the atmosphere, at least in the southern plains.