Water vapour in the lower atmosphere of Venus: a new analysis of optical spectra measured by entry probes

The first reliable identification of water vapour in the lower atmosphere of Venus was made in 1978 from optical spectra of sunlight penetrating through the planetary cloud cover. These spectra were measured by the Venera 11 entry probe. Later, similar measurements were carried out by the Venera 13 and 14 probes (1982). Since 1984 Earth-based observations of the near-infrared emission from Venusʹ night-side have allowed remote sounding of the lower atmosphere, including determination of the water vapour abundance. The results of these two types of optical observations proved to be inconsistent in vertical distribution of H2O. The goal is to reanalyse the Venera 11, 13, and 14 data to explore this contradiction. The data have been reinterpreted using recently available spectroscopic data bases with a line-by-line procedure for calculation of the gaseous opacity and a discrete ordinate radiative transfer program to generate monochromatic intensity spectra. Contrary to the previous interpretation of these data, the H2O mixing ratio was found to be nearly constant and equal to 30 ± 10 ppm in the altitude range of 5–60 km. It seems likely that the H2O mixing ratio varies slightly from 40 ppm in the clouds down to a minimum of 20 ppm at 10–20 km (model A), although another possibility of a constant 30 ppm profile (model B) also cannot be discarded. Both models are in good agreement with Earth-based observations within experimental errors of both datasets, so the earlier contradictions are eliminated. Also it is found that there is no minimum of H2O mixing ratio near the surface; furthermore, it probably increases up to 50–70 ppm at altitudes below 5 km.