The near-infrared nitric oxide nightglow in the upper atmosphere of Venus

The v' = 0 progressions of the C -> X and A -> X band systems of nitric oxide dominate the middle-UV spectrum of the night-time upper atmospheres of the Earth, Mars, and Venus. The C( 0)-> A( 0) + h nu radiative transition at 1.224 mu m, the only channel effectively populating the A( 0) level, must therefore occur also. There have been, however, no reported detections of the C( 0) -> A( 0) band in the atmospheres of these or any other planets. We analyzed all available near-infrared limb observations of the dark-side atmosphere of Venus by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument on the Venus Express spacecraft and found 2 unambiguous detections of this band at equatorial latitudes that seem to be associated with episodic events of highly enhanced nightglow emission. The discovery of the C( 0)-> A( 0) band means observations in the 1.2-1.3 mu m region, which also contains the a( 0)-> X( 0) emission band of molecular oxygen, can provide a wealth of information on the high-altitude chemistry and dynamics of the Venusian atmosphere.