Analysis of CO2 non-LTE emissions at 4.3μm in the Martian atmosphere as observed by PFS/Mars Express and SWS/ISO

Daytime nadir spectra of the Martian atmosphere taken by the planetary Fourier spectrometer/Mars Express (PFS) and by the infrared space observatory (ISO) in the 4.3μm region are analyzed using a theoretical non-LTE model of the CO2 infrared emissions in that atmosphere. Averaged spectra from these instruments show a double peak structure which can be explained by the strong solar pumping at 2.7μm of the (1001) and (0201) vibrational states of the major CO2 isotope. After the initial absorption, these CO2 states relax radiatively by emission in the 4.3μm second hot bands, whose R- and P-branches produce such structure. It is shown that an LTE atmosphere could not produce such structure. A good fit to the measured spectra is obtained, particularly with PFS, using the currently assumed non-LTE parameters without special adjustments. Analysis of individual bands contributions, their dependence with the thermal structure and solar zenith angle, and the simulation of the altitude variation of the different non-LTE features are presented and discussed. These results confirm the prediction of the non-LTE model and have allowed to identify the main spectral features observed in PFS and ISO data. The dominant double maxima are due to the second hot bands while the individual lines come from the fundamental band. Also the Q-branch of the first hot band and the absence of emission lines in the center of the fundamental band in the PFS spectrum are well reproduced by the simulations.