Observing the surface of Venus with VIRTIS on Venus Express

We will report about first observation of the surface of Venus using the atmospheric windows located at 1.02, 1.10, 1.18 μm with the M-channel of VIRTIS on Venus Express. Observations of the southern hemisphere obtained directly after orbit insertion and during the first phase of science observations show radiance variations correlated with large scale topographic features on the surface. Stacking of the available data allows to procedure a very first tentative mosaic of the southern hemisphere. While the atmospheric windows show little CO2 absorption, on its way through the atmosphere the thermal radiation is modified by scattering and absorption by clouds. Variations in the optical thickness of the clouds modulate the spatial distribution of upwelling radiation. Multiple reflections between surface and clouds generally wash out image contrast from surface emissivity. The surface data presented here has been obtained using our processing procedure (Mueller et al, this meeting) which reduces the cloud induced variation and enhances thereby the signal from the surface. The procedure corrects also for various atmospheric effects, incl. limb darkening, scattered sunlight, etc. During the mission the algorithm will be refined based on the data returned from the different instruments on Venus Express. The next step will be to search for relative emissivity variation and to map their correlation with geological units. The final goal is to derive maps of the absolute surface emissivity leading to a classification of surface types. Furthermore the mapping of the variability of the surface temperature can be a direct indicator for active volcanism. Therefore VIRTIS will be the first instrument to routinely monitor the surface of Venus for volcanic activity.