Venus Airglow Measurements Obtained by the Mercury Atmospheric and Surface Composition Spectrometer During MESSENGER's Second Venus Flyby

During MESSENGER's second flyby of Venus on 5 June 2007, the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) conducted several different observations of the Venus airglow using the Ultraviolet- Visible Spectrometer (UVVS) component of the instrument. On the inbound leg and crossing into the sunlit hemisphere, measurements of H Lyman α (121.6 nm) and O (130.4 nm) were obtained. Once the MASCS line of sight crossed the terminator into the night hemisphere, measurements of the NO δ and γ bands and of the O2 Herzberg II band system were added. Because the MASCS instrument is a scanning spectrometer, these measurements were obtained in series, with the cycle repeating until the line of sight moved off of the surface by several hundred kilometers. The sequence then returned to H and O measurements only on the outbound leg. The rapid speed of the flyby and design of MASCS (optimized for Mercury orbit) limited the number of measurements that were possible. Nevertheless, a fair number of spectra were obtained and represent an opportunity not only to calibrate MASCS against other measurements but also to provide additional, perhaps unique, data on the Venus airglow. We present an analysis of these spectra and place them in the context of previous measurements by the Pioneer Venus Orbiter Ultraviolet Spectrometer (H, O, and NO), the Venera orbiters (O2), and ground-based telescopes (O2) as well as continuing measurements by the Venus Express spacecraft.