Water Photolysis and Its Contributions to the Hydroxyl Dayglow Emissions in the Atmospheres of Earth and Mars.

Airglow is a well-known phenomenon in the Earth's upper atmosphere, which arises from the emissions of energetic atoms and molecules. The Meinel band emission from high vibrationally excited OH(X) radicals is one of the more important contributors to the airglow from the mesosphere/lower thermosphere. The H + O3 reaction has long been regarded as the dominant source of these OH(X, high v) radicals. Here we demonstrate that vacuum ultraviolet (VUV) photolysis of water vapor at λ ∼ 112.8 nm represents another source of exceptionally highly vibrationally excited OH(X) radicals, with a nascent vibrational state population distribution that maximizes at v = 9 and extends to at least the v = 15 level. Atmospheric chemistry modeling indicates that OH(X, high v) radicals from H2O photolysis might be detectable in the OH Meinel band dayglow in the upper atmosphere of Earth and should dominate the corresponding emission from the Martian atmosphere. VUV photolysis of H2O also produces electronically excited OH(A) radicals, and simultaneous detection of emissions from OH(X, high v) and OH(A) is shown to offer a route to identifying high-oxygen exoplanetary atmospheres.