Lower atmosphere minor gas abundances as retrieved from Venus Express VIRTIS-M-IR data at 2.3 µm

Abstract

Minor gas abundances in the lower atmosphere of Venus׳ southern hemisphere are investigated using spectroscopic nightside measurements recorded by the Visible and InfraRed Thermal Imaging Spectrometer aboard ESA’s Venus Express mission in the moderate spectral resolution infrared mapping channel (VIRTIS-M-IR, 1–5µm, FWHM=17nm). The entire usable data archive is utilized including only radiation spectra sampled at long detector exposure times (≥3.3s) during eight Venus solar days between April 2006 and October 2008. Combined radiative transfer and retrieval techniques (Haus et al., 2013; Haus et al., 2014) are applied for a simultaneous determination of total cloud opacity and H2O, CO, and OCS abundances from the 2.3µm atmospheric transparency window that sounds the altitude range between about 30 and 45km. A wavelength-dependent CO2 opacity correction is considered.Zonal averages of CO abundances at 35km increase by about 35% from (22.9±0.8) ppmv at equatorial latitudes to (31.0±2.1) ppmv at 65°S and then decrease to (29.4±2.4) ppmv at 80°S The±figures refer to the statistical variability of retrieved abundances. In accordance with earlier results, the observed latitudinal variation of tropospheric CO is consistent with a Hadley cell-like circulation. Dawn side CO abundances at high latitudes are slightly smaller than dusk side values by about 7%. The latitudinal distribution of OCS at 35km is anticorrelated with that of CO, ranging from about (1.15±0.2) ppmv at 65°S to (1.60±0.2) ppmv at low latitudes (poleward decrease of 28%). Zonal averages of H2O abundances near 35km slightly decrease toward the South Pole by about 10%, and the hemispheric average is (32.0±1.3) ppmv. A significant local time dependence of OCS and H2O is not observed. Detailed analyses of individual spectrum retrieval errors for different atmospheric models reveal that CO abundance results are reliable (error 4–7%), while H2O and OCS results have lower confidence (errors 30–47% and 41–86%, respectively). SO2 abundances cannot reliably be retrieved from VIRTIS-M-IR spectra.