The Thermal Profile and Water Abundance in the Venus Mesosphere from H2O and HDO Millimeter Observations

The 183.310-GHz water vapor transition has been observed in the Venus atmosphere for the first time. The detection was made in January 1991, using the 30-m IRAM radiotelescope. A spectrum of Saturn, recorded during the same night, was used to remove the contribution due to terrestrial mesospheric water. Using the depth and the FWHM of the observed absorption line, we derive a water mixing ratio of 1.0 (+ 1.0, -0.5) ppm above the clouds, and a temperature of 140 (±10) K at the level z = 95 km ( P = 0.06 mb). In July 1993, we reobserved the HDO transition at 225.987 GHz, which we first detected in August 1990 (T. Encrenaz et al. 1991, Astron. Astrophys. 246, L63-L66). The new set of data was obtained at the Caltech Submillimeter Observatory (CSO), with improved signal-to-noise and spectral resolution with respect to previous measurements. From the 1993 data we derive a water mixing ratio of 7.0 (+5.0, -4.0) ppm above the clouds, and a temperature higher than 145 K at the z = 95-km level. The water abundance is derived assuming a deuterium enrichment factor of 120 with respect to the terrestrial value (C. de Bergh et al . 1991, Science 251, 547-549). If the deuterium enrichment factor is close to 160, as measured by G. L. Bjoraker et al . (1992, Bull. Am. Astron. Soc. 24, 995-995), the difference between the H 2 O measurements is reduced, but the data might still suggest some possible fluctuation in the water vapor content above the clouds between 1991 and 1993.