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 t he NIMS instrument at the time of the Galileo flyby of Venus, is analy sed to infer the properties of the upper cloud boundary. From the glob al maps of Venus at fixed wavelengths, the limb darkening of the flux is measured at several latitudes, within each infrared channel. By usi ng the nominal Pioneer Venus thermal profile, these data give access t o two parameters the cloud deck temperature and the cloud scale height . It is verified independently, from the NIMS spectra, that this therm al profile is consistent with all the NIMS observations, and that the thermal structure does not vary significantly in the latitude range (2 5 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 acc uracy of about 15%, taking into account the various sources of theoret ical and observational uncertainties. At higher latitudes, the tempera ture profile becomes more isothermal and the presented method to retri eve His no longer valid.