Ground‐based near‐infrared observations of the Venus nightside: The thermal structure and water abundance near the surface

Abstract

We used ground‐based near‐infrared (NIR) observations of thermal emission from the Venus nightside to determine the temperature structure and 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 μm originates primarily from the surface and lowest scale height (∼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 scattering by the 90‐bar CO2 atmosphere and Mie scattering by the H2SO4 clouds attenuate this emission, but add little to its spectral dependence. Surface topography also modulates this NIR thermal emission because high‐elevation regions are substantially cooler and emit less thermal radiation than the surrounding plains. These contributions to the emission are clearly resolved in moderate‐resolution (λ/Δλ ∼ 400) spectral image cubes of the Venus nightside acquired with the infrared imaging spectrometer (IRIS) on the Anglo‐Australian Telescope (AAT) in 1991. To analyze these observations, we used a radiative transfer model that includes all of the radiative processes listed above. Synthetic spectra for several topographic elevations were combined with Pioneer Venus altimetry data to generate spatially resolved maps of the NIR thermal emission. Comparisons between these synthetic radiance maps and the IRIS observations indicate no near‐infrared signature of the surface emissivity differences seen at microwave wavelengths by the Magellan orbiter. Assuming constant surface emissivity in the near‐infrared, we derive nightside averaged temperature lapse rates of −7 to −7.5 K/km in the lowest 6 km. These lapse rates are smaller 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 ratio 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 surface. There is no evidence for H2O mixing ratios that decrease with altitude, like those inferred from the Pioneer Venus large probe mass spectrometer [Donahue and Hodges, 1992a] or the Venera 11 and 12 Lander spectrophotometers [Moroz, 1983].