The 1.10- and 1.18-μm nightside windows of Venus observed by SPICAV-IR aboard Venus Express
Abstract
Observations of the 1.10- and 1.18-μm nightside windows by the SPICAV-IR instrument aboard Venus Express were analyzed to characterize the various sources of gaseous opacity and determine the H 2O 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 CO 2 absorption in the high-wavelength wing of the 1.18-μm 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-μm radiances with surface elevation as observed by the VIRTIS-M instrument aboard Venus Express. A constant absorption coefficient of 0.7 ± 0.2 × 10 -9 cm -1 am -2 best reproduces the observed variation. We compared spectra calculated with different CO 2 and H 2O line lists. We found that the CDSD line list lacks the 5 ν1 + ν3 series of CO 2 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-μm windows, this result provides strong evidence for a uniform H 2O profile below 40 km, in agreement with chemical models.
- Publication:
-
Icarus
- Pub Date:
- November 2011
- DOI:
- 10.1016/j.icarus.2011.08.025
- Bibcode:
- 2011Icar..216..173B