Carbon Monoxide (CO)

The primary source of CO in the atmosphere of Venus is the photo-dissociation of CO2 by solar UV at altitudes higher than 120 km. The mixing ratio of CO was measured by the Pioneer Venus gas chromatograph [Oyama et al., 1980] at different altitudes in the lower atmosphere (between 22 and 62 ppm at 52 km, 30±18 ppm at 42 km and 20±3 ppm at 22 km). The gas chromatograph on Venera 12 [Gel'man et al., 1980] confirmed the low value found below 42 km (28±7 ppm). Connes et al. [Connes et al., 1968] reported a value of 45±10 ppm at 64 km from Earth based observations in the near infrared. This value was corrected to 51±1 ppm by Young (1972) who reanalyzed the spectra recorded by Connes et al. (1968). These measurements seem to indicate the presence of a gradient in the mixing ratio of CO at least in the altitude range sounded.  This was confirmed by observations of microwave lines of CO, which yielded CO mixing ratio for altitudes between 75 and 105 km. The CO mixing ratio increases from 55 ppmv at 75 km, to 130 ppmv at 85 km and 200 to 1,000 ppmv at 105 km. Moreover it was shown that CO exhibits a significant diurnal variation but also strong year to year variations [Clancy and Muhleman, 1991; Clancy et al., 2003; Gurwell et al., 1995]. Ground-based observations of the night side of Venus [Marcq et al., 2005; Marcq et al., 2006] have reported that the CO abundances in the lower atmosphere (below the clouds) showed a pronounced latitudinal enhancement of more than 10% when going towards the poles. Observations by the VIRTIS-M instrument on Venus Express [Irwin et al., 2008] have shown that there was little spatial distribution of CO just above the cloud (approximately 65-70 km) at mid-latitudes, with abundances of the order of 40±10 ppm, with higher values at the poles, consistent with rapid downwelling bringing CO-rich air from higher altitudes.


MATLAB Handle Graphics

Vertical profiles of the CO density (left) and volume mixing ratio in ppm (right) obtained during the three occultations 341 ( ─ : 28/03/2007; distance to the limb at 65 km tangent height: 3346 km; 82 N), 356 ( …. : 12/04/2007; distance to the limb: 3298 km; 84 N), and 366 ( - - - : 22/04/2007; distance to the limb: 3898 km; 73 N).

For comparison, the solid line with dots corresponds to the CO in the VENUSREF model. Error bars correspond to the fitting error on the single fitted parameter, i.e. the density (vmr) in the corresponding layer, and does not take into account the errors on the densities (vmr) in the upper layers. (credits: BISA)

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Relevant Publications

Composition of the Venus mesosphere sosphere measured by SOIR on board Venus Express (pdf - 743 KB)
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