Water vapor abundance near the surface of Venus from Venus Express/VIRTIS observations

Nightside observations of the 1.18‐μm atmospheric window by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) aboard the Venus Express spacecraft were analyzed to measure and map the water vapor abundance in the lower atmosphere. Thermal emission in this window originates partly from the surface and partly from the first scale height (0–15 km) of the atmosphere. Constraints on the CO2 continuum absorption, which is the dominant source of gaseous opacity in the window, were obtained from the variation of the 1.185‐μm intensity with surface elevation. An absorption coefficient of 1 ± 0.4 × 10−9 cm−1 amagat−2 best fits the observed variation. We retrieved a water vapor mole fraction of 44 ± 9 ppm from various selections of VIRTIS spectra in the southern hemisphere, in agreement with previous analyses of the nightside emission. This value is somewhat larger than that previously determined at higher altitudes from the 2.3‐ and 1.74‐μm nightside windows, but the error bars still allow a constant with height H2O mole fraction from the surface up to 40 km. Using the intensity ratio in the two wings of the 1.18‐μm window as a proxy, we searched for horizontal variations of the H2O abundance in various VIRTIS observational sequences. We derived stringent upper limits for any possible latitudinal variations on the night side: ±1.5% in the range 60°S–25°N and ±3% for the broader range 80°S–25°N. The lack of detectable latitudinal variations is consistent with a constant with height water profile in the lower atmosphere and probably precludes any strong concentration gradient near the surface.