Water vapor abundance in Venusʹ middle atmosphere from Pioneer Venus OIR and Venera 15 FTS measurements

The Pioneer Venus Orbiter Infrared Radiometer and Venera 15 Fourier Transform Spectrometer observations of thermal emission from Venusʹ middle atmosphere between 10° S and 50° N have been independently re-analyzed using a common method to determine global maps of temperature, cloud optical depth, and water vapor abundance. The spectral regions observed include the strong 15 μm carbon dioxide band and the 45 μm fundamental rotational water band. The different spatial and spectral resolutions of the two instruments have necessitated the development of flexible analysis tools. New radiative transfer and retrieval models have been developed for this purpose based on correlated-k absorption tables calculated with up-to-date spectral line data. The common analysis of these two sets of observations has hence been possible for the first time. From the PV OIR observations, the cloud-top unit optical depth pressure showed a minimum of ∼ 110 ± 10   mbars in the evening equatorial region and a maximum of ∼ 160 ± 12   mbars in the morning mid-latitude regions. From the Venera 15 FTS spectra, the cloud-top pressure was found to increase from morning values of ∼ 120 ± 10 to 200 ± 30   mbars in the late afternoon/early evening region. The cloud-top water vapor abundances observed by the PV OIR instrument were found to fluctuate from 10 ± 5   ppm at night up to 90 ± 15   ppm in the equatorial cloud-top region shortly after the sub-solar point. The mean Venera 15 FTS water vapor abundances were found to be 12 ± 5   ppm with only a slight enhancement over the equatorial latitude bands and no clear day–night distinction. The common analysis of these two sets of observations broadly validates previously published individual findings. The differences in the retrieved atmospheric state can no longer be attributed to radiative transfer modeling bias and suggest significant temporal variability in the middle atmosphere of Venus.