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)+hν radiative transition at 1.224 μm, 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 μm 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.