Ultraviolet photograph of night airglow on Venus, taken in 1979 by the Pioneer-Venus Orbiter's ultraviolet spectrometer at a wavelength of 198 nanometres. The white crescent at top right is the planet's sunlit side. The glow on the night-time side, coded for intensity ranging from yellow (bright) to black (faint) is caused by atoms of nitrogen & oxygen combining together fluorescently to produce nitric oxide. The atoms have been carried round by high winds from the daytime hemisphere, where they were produced by the action of sunlight on molecules of carbon dioxide & nitrogen. The ultraviolet glow gives clues to the high-altitude winds in the Venusian atmosphere. Reference: Figure 2.17 in THE NEW ASTRONOMY.
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