The 1.10- and 1.18-μm nightside windows of Venus observed by SPICAV-IR aboard Venus Express

Observations of the 1.10- and 1.18-mu m nightside windows by the SPICAV-IR instrument aboard Venus Express were analyzed to characterize the various sources of gaseous opacity and determine the H2O mole fraction in the lower atmosphere of Venus. We showed that the line profile model of Afanasenko and Rodin (Afanasenko, T.S., Rodin, A.V. [2007]. Astron. Lett. 33, 203-210) underestimates the CO2 absorption in the high-wavelength wing of the 1.18-mu m window and we derived an empirical lineshape that matches this wing well. An additional continuum opacity is required to reproduce the variation of the 1.10- and 1.18-mu m radiances with surface elevation as observed by the VIRTIS-M instrument aboard Venus Express. A constant absorption coefficient of 0.7 +/- 0.2 x 10(-9) cm(-1) am(-2) best reproduces the observed variation. We compared spectra calculated with different CO2 and H2O line lists. We found that the CDSD line list lacks the 5 nu(1) + nu(3) series of CO2 bands, which provide significant opacity in Venus' deep atmosphere, and we have constructed a composite line list that best reproduces the observations. We also showed for the first time that HDO brings significant absorption at 1140-1190 nm. Using the best representation of the atmospheric opacity we could reach, we retrieved a water vapor mole fraction of 30(-5)(+10) ppmv, pertaining to the altitude range 5-25 km. Combined with previous measurements in the 1.74- and 2.3-mu m windows, this result provides strong evidence for a uniform H2O profile below 40 km, in agreement with chemical models. 2011 Elsevier Inc. All rights reserved.