A latitudinal survey of CO, OCS, H 2 O, and SO 2 in the lower atmosphere of Venus: Spectroscopic studies using VIRTIS‐H

The high‐resolution channel ( R ≃ 2000) of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument (VIRTIS‐H) aboard Venus Express has provided numerous spectra of the nightside infrared thermal emission in the 2.3‐ μ m window. Mixing ratios of various minor species in the 30–40 km range could therefore be inferred using this spectral window at higher latitudes accessible to the spacecraft but which cannot be observed from Earth. The previously known enhancement in carbon monoxide (CO) toward high latitudes is confirmed and extended up to 60° with a mixing ratio varying from 24 ± 3 to 31 ± 2 ppmv at 36 km. Measurements of carbonyl sulfide (OCS) also agree with the previously suspected latitudinal variations that are anticorrelated with those of CO, ranging between 2.5 ± 1 and 4 ± 1 ppmv at 33 km. New constraints were also derived on the mean abundance of water vapor (H 2 O, 31 ± 2 ppmv) and sulfur dioxide (SO 2 , 130 ± 50 ppmv) in the probed altitude range. CO and OCS variations are interpreted as caused by large‐scale vertical motions, an explanation under current testing by various chemical and dynamical modeling. In such a case, these variations may help constrain the chemical time scale of those species in the lower troposphere.