Variations of carbon monoxide in the martian lower atmosphere

Abstract

Our observations of variations of CO on Mars by means of the ground-based spatially-resolved high-resolution spectroscopy (Krasnopolsky, V.A. [2003]. J. Geophys. Res. 108(E2), 5010; Krasnopolsky, V.A. [2007]. Icarus 190, 93–102) have been significantly improved using the 13CO lines near 4148cm−1 and the CO2 lines near 4570cm−1. These lines are of optimal strength, of low sensitivity to variations of temperature, and covered by the ATMOS solar spectrum that makes it possible to use the synthetic spectra technique for retrieval of CO and CO2 to get CO mixing ratios. The CO2 line strengths from Toth et al. (2008) were also essential to improve accuracy of the results.The 13CO/CO ratio of 1.023 times the terrestrial carbon isotope ratio was calculated using the known 13CO2/CO2=1.046 in the martian atmosphere (Webster, C.R., et al. [2013]. Science 341, 260–263), the photo-induced isotope fractionation (Miller, C.E., Yung, Y.L. [2000]. J. Geophys. Res. 105(D23), 29039–29051) in the CO2 photolysis, and isotope fractionation in the reaction between CO and OH (Feilberg, K.L., Johnson, M.S., Nielsen, C.J. [2005]. Phys. Chem. Chem. Phys. 7, 2318–2323).The observations were conducted at LS=60°, 89°, 110°, and 145° and extend over the maximum of CO in the southern hemisphere during the northern summer. The CO mixing ratio was observed to be constant over the 55°S–90°N latitudinal range to within 7%, for each observed LS period. Therefore our observations show that the enrichment of incondensable gases by condensation of CO2 in the southern polar regions does not significantly extend to the middle and low latitudes. This behavior agrees with the Mars Climate Database (Lefevre, F., Forget, F. [2009]. Nature 460, 720–722), whereas most other observations exhibit much larger latitudinal gradients and seasonal variations. Our measurements do not show the CO depletion at high northern latitudes predicted by MCD of ∼20% at LS≈60–150° and observed as much stronger by MRO/CRISM (Toigo, A.D., et al. [2013]. J. Geophys. Res. 118E, 89–104).The retrieved global and annually mean CO abundance is equal to 700ppm on Mars, which is also smaller than many recent results by a factor of ∼1.4.