New nitric oxide (NO) nightglow measurements with SPICAM/MEx as a tracer of Mars upper atmosphere circulation and comparison with LMD-MGCM model prediction: Evidence for asymmetric hemispheres

We report observations of NO nightglow with the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) experiment on board the Mars Express (MEx) spacecraft. NO molecules emit an ultraviolet photon when N and O atoms (produced at high altitude in the thermosphere) recombine. Therefore, this emission is a tracer of the atmospheric dynamics in the lower thermosphere where O and N atoms are produced, and below, in the altitude region 50-100 km where the emission is detected. A new retrieval method has been developed to analyze the measurements from this instrument in the stellar occultation mode without slit and retrieve the absolute brightness of the emission. We present the results from the processing of more than 2000 orbits, providing the first global latitude-season distribution of the emission, established over three Martian years. The results are globally consistent with previously available measurements of dedicated limb nightglow obtained during the first Martian year of MEx (MY27). We compared the ensemble of both data sets with the predictions of the Laboratoire de Meteorologie Dynamique Mars General Circulation Model (LMD-MGCM), with the addition of the full chemistry of N atoms. We find an overall agreement between the observed and modeled airglow intensities, but discrepancies are also found. The frequency and magnitude of the NO airglow observations show important asymmetries between the Northern and the Southern Hemispheres. There is no detection of emission near the poles during equinox conditions, while the model predicts that it should be most intense because of a circulation with two descending branches at the poles.