Observations of O2 1.27 μm Nightglow Emission At Mars With Omega/MEX and SPICAM/MEX : Comparison With GCM Model Predictions
Résumé
The first detection in the atmosphere of Mars of the night side O2(a1Δg) emission at 1.27 mm from limb observations of the OMEGA imaging spectrometer on board Mars Express was reported earlier (Bertaux et al., JGR, 2012). The emission, detected in 3 cases out of 40 observations is due to recombination in a downwelling air parcel of O atoms produced by photodissociation of CO2 on the dayside in the upper atmosphere (O+O+M® O2*+M), and not from ozone UV photodissociation, as is often seen on the day side. This emission was also detected with SPICAM/MEX. It is similar to the strong O2 Venus nightglow, which traces the transport of thermospheric air from the day side to the night side. However, in the case of Mars, it traces a meridional transport from the summer pole to the winter pole, along a single Hadley's cell, similar to the terrestrial case. This can be explained in the frame of a general circulation model (GCM) of Mars. As predicted by the GCM, all positive observations were obtained at high latitudes, during the winter night or near the equinox. An interesting feature of the model is that it predicts that near both equinoxes, this emission may happen simultaneously at both pole regions, showing that the circulation is then characterized by two Hadley's cells, ascending at equator and downwelling at both poles. The search for such a situation in Mars Express data will be reported. The model is validated, which simulates the large Hadley cell characterizing the meridional circulation, ascending from the summer pole and descending to the winter pole. This new emission is tracing uniquely a downward advection transport mechanism, and therefore its detailed study will provide important constraints on the overall aeronomy and dynamics of Mars.