THE UPPER CLOUDS OF VENUS - DETERMINATION OF THE SCALE HEIGHT FROM NIMS-GALILEO INFRARED DATA

The 3-5 mu m thermal emission of the nightside of Venus, recorded by the NIMS instrument at the time of the Galileo flyby of Venus, is analysed to infer the properties of the upper cloud boundary. From the global maps of Venus at fixed wavelengths, the limb darkening of the flux is measured at several latitudes, within each infrared channel. By using the nominal Pioneer Venus thermal profile, these data give access to two parameters the cloud deck temperature and the cloud scale height. It is verified independently, from the NIMS spectra, that this thermal profile is consistent with all the NIMS observations, and that the thermal structure does not vary significantly in the latitude range (25 degrees S, 30 degrees N). Within this range, the cloud scale height is found to be constant with latitude, and is is H=5.2 km, with an accuracy of about 15%, taking into account the various sources of theoretical and observational uncertainties. At higher latitudes, the temperature profile becomes more isothermal and the presented method to retrieve His no longer valid.