Remote sensing of Venus’ lower atmosphere from ground-based IR spectroscopy: Latitudinal and vertical distribution of minor species

We present an investigation of the lower atmosphere in the 25–40 km altitude range from data of Venus’ night side in the 2.3- μ m region. We used observations recorded at the NASA/IRTF in August 2004 with the SpeX spectro-imager. The previous conclusions of Marcq et al. [2005, Latitudinal variations of CO and OCS in the lower atmosphere of Venus from near-infrared nightside spectro-imaging, Icarus 179, 375–386] about CO and OCS abundance latitudinal variations are confirmed with a better accounting for cloud opacity variations; in particular, an enhancement of 12 ± 4 % in CO at 36 km is found at 40 ∘ S with respect to the equator. There is also a depletion of OCS southward of 20 ∘ S, that may be correlated with the CO increase. Information on the vertical gradients of the CO and OCS profiles was derived, yielding the following mixing ratios in the probed altitude range: CO ( 30 km ) = 22.5 ± 3.5 ppm , CO ( 36 km ) = 24 ± 2 ppm ; OCS ( 30 km ) = 5 – 20 ppm , OCS ( 36 km ) = 0.55 ± 0.15 ppm . The study was also extended to H 2 O and no latitudinal variations are found beyond our error bars ( 26 ± 4 ppm at 35 km ).