SEARCH FOR SPATIAL VARIATIONS OF THE H2O ABUNDANCE IN THE LOWER ATMOSPHERE OF VENUS FROM NIMS-GALILEO

The spectroscopic data of the Near-Infrared Mapping Spectrometer (NIMS ), recorded during the Galileo flyby of Venus, are analysed to retriev e the water vapour abundance variations in the lower atmosphere of Ven us at night. The 1.18 mu m spectral window, which probes altitude leve ls below 20 km, is used for this purpose. Constraints on the CO2 conti nuum and far-wing opacity from existing ground-based high-resolution o bservations are included in the modelling of the NIMS spectra. The NIM S measurements can be fitted with a water vapour mixing ratio of 30+/- 15 ppm, in agreement with analyses of ground-based nightside observati ons. The water vapour abundance shows no horizontal variations exceedi ng 20% over a wide latitude range (40 degrees S, 50 degrees N) on the nightside of Venus. Within the same selection of NIMS spectra, a large enhancement in the O-2 fluorescence emission at 1.27 mu m is observed at a latitude of 40 degrees S, over a spatial area about 100 km wide.