GROUND-BASED NEAR-INFRARED OBSERVATIONS OF THE VENUS NIGHTSIDE - THE THERMAL STRUCTURE AND WATER ABUNDANCE NEAR-THE-SURFACE

We used ground-based near-infrared (NIR) observations of thermal emiss ion from the Venus nightside to determine the temperature structure an d water vapor distribution between the surface and the 6-km level. We show that emission from spectral windows near 1.0, 1.1, and 1.18 mu m originates primarily from the surface and lowest scale height (similar to 16 km). These windows include absorption by weak H2O and CO2 lines and by the far wings of lines in strong nearby CO2 bands. Rayleigh sc attering by the 90-bar CO2 atmosphere and Mie scattering by the H2SO4 clouds attenuate this emission, but add little to its spectral depende nce. Surface topography also modulates this NIR thermal emission becau se high-elevation regions are substantially cooler and emit less therm al radiation than the surrounding plains. These contributions to the e mission are clearly resolved in moderate-resolution (lambda/Delta lamb da similar to 400) spectral image cubes of the Venus nightside acquire d with the infrared imaging spectrometer (IRIS) on the Angle-Australia n Telescope (AAT) in 1991. To analyze these observations, we used a ra diative transfer model that includes all of the radiative processes li sted above. Synthetic spectra for several topographic elevations were combined with Pioneer Venus altimetry data to generate spatially resol ved maps of the NIR thermal emission. Comparisons between these synthe tic radiance maps and the IRIS observations indicate no near-infrared signature of the surface emissivity differences seen at microwave wave lengths by the Magellan orbiter. Assuming constant surface emissivity in the near-infrared, we derive nightside averaged temperature lapse r ates of -7 to -7.5 K/km in the lowest 6km. These lapse rates are small er and indicate much greater static stability than those inferred from earlier measurements and greenhouse models (-8 to -8.5 K/km) [Seiff; 1983]. An acceptable fit to the data was obtained with an H2O mixing r atio profile which increases from 20 ppmv at the cloud base to 45 ppmv at 30 km, and then remains constant between that altitude and the sur face. There is no evidence for H2O mixing ratios that decrease with al titude, like those inferred from the Pioneer Venus large probe mass sp ectrometer [Donahue and Hedges, 1992a] or the Venera 11 and 12 Lander spectrophotometers [Moroz, 1983].