Observations of CO in the atmosphere of Mars in the (2-0) vibrational band at 2.35 microns

Following our high-resolution infrared observations of CO in the atmosphere of Mars in 1988 and 1989 at 4.7 μm (Billebaud et al., 1992), we recorded new spectra of CO: one covering the whole disk of the planet in 1990 and 4 spectra corresponding to 4 different locations on the planet in 1991. All these spectra were recorded in the (2-0) vibrational band at 2.35μm. These data allow us to measure the CO abundance and to search for possible middle-scale spatial variations of this abundance in the case of the 1991 spectra. The CO mixing ratio derived from the 1990 data is in good agreement with the values we obtained in 1988 and 1989 (Billebaud et al., 1992), showing a great stability over a period of 3 years, with a value of the CO mixing ratio remaining in the range of 4.2-8.5 × 10^-4. The results we obtained with the 1991 data also seem to comfort the stability of the CO mixing ratio, although the possible range is somewhat larger (5.5-11.5 × 10^-4), This common CO mixing ratio range for the four locations on the planet then tends to exclude the presence of any significant horizontal variations of the CO mixing ratio, even if, from our data, we cannot firmly rule them out.