| Summary: | International audience The OMEGA hyperspectral imager on board Mars Express is primarily dedicated to day side observations of both the surface and the atmosphere of Mars. However, observations on the night side were also performed occasionally, and in particular above the planetary limb. Along its 6 years of operation to date, OMEGA has acquired hundreds of night side limb profiles of the Martian atmosphere, at a variety of seasons and latitudes. In some of them, the O2 emission at 1.27µm is observed. In one particular case acquired at Ls= 120° and 76.5° S latitude, the peak intensity reached 14 MegaRayleigh at an altitude of 42 km. We interpret this emission as originating from the recombination of oxygen atoms, similar to the corresponding emission observed in the Venus night side. Specifically, for both Venus and Mars, CO2 and N2 are photo-dissociated during the day in the thermosphere; O and N atoms are transported by thermospheric circulation to the night side, where air is descending. During descent, O2 recombines and emits diagnostic lines, detected by OMEGA in the NIR. This nightglow is similar to that of NO previously detected by SPICAM/Mars Express in the UV. The newly observed recombination O2 emission is produced in the night side by a totally different mechanism than the Martian day side emission at 1.27 µm, due to photo-dissociation of ozone. These first observations of O2 recombination are compared with predictions of a 3D GCM developed at LMD, showing that downward transport from the thermosphere is occurring mainly in the polar night. The OMEGA observations will help to quantify the atmospheric downward vertical transport from the thermosphere. It is worth to note that air coming from the thermosphere is void of methane, which lifetime at high altitudes is one week, instead of 200 years at ground level.
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