CO2 non‐LTE limb emissions in Mars' atmosphere as observed by OMEGA/Mars Express

We report on daytime limb observations of Mars upper atmosphere acquired by the OMEGA instrument on board the European spacecraft Mars Express. The strong emission observed at 4.3 μm is interpreted as due to CO 2 fluorescence of solar radiation and is detected at a tangent altitude in between 60 and 110 km. The main value of OMEGA observations is that they provide simultaneously spectral information and good spatial sampling of the CO 2 emission. In this study we analyzed 98 dayside limb observations spanning over more than three Martian years, with a very good latitudinal and longitudinal coverage. Thanks to the precise altitude sounding capabilities of OMEGA, we inferred the vertical profiles of the non-LTE emission at each wavelength and we studied their dependence on several geophysical parameters, such as the solar illumination and the tangent altitude. The dependence of the non-LTE emission on solar zenith angle and altitude follows a similar behavior to that predicted by the non-LTE model. According to our non-local thermodynamic equilibrium model (Non-LTE), the pressure level where the peak of the emission is found remains constant at ∼0.03±0.01 Pa, and we have shown with SPICAM stellar occultation retrievals that the seasonal variations of constant pressure level altitudes correlate well with the variations of the OMEGA peak emission altitudes, although the exact pressure level can not be defined with the SPICAM nighttime data. The tangent altitude of this atmospheric layer depends on the structure of the whole atmosphere below, and represents a strong validation tool for atmospheric models. We thus compared the altitude of OMEGA peak emission with the tangent altitude of the 0.03 Pa level predicted by the LMD-Mars Global Circulation Model. However, the peak emission altitudes from OMEGA present a much larger variability than the tangent altitude of the 0.03 Pa level predicted by the GCM; this variability could be possibly due to unresolved atmospheric waves. Further studies using this strong CO 2 limb emission data are proposed.