Visible and Near-Infrared Nightglow of Molecular Oxygen in the Atmosphere of Venus

Abstract

The Herzberg II system of O₂ has been a known feature of Venus’ nightglow since the Venera 9 and 10 orbiters detected its c(0)–X(v″) progression more than 3 decades ago. We search for its emission at 400 nm–700 nm in spectra obtained with the VIRTIS instrument on Venus Express. Despite the weakness of the signal, integration over a few hours of limb observations of the planet’s upper atmosphere reveals the unambiguous pattern of the progression. The selected data sample mainly the northern latitudes within a few hours of local midnight. The emission is ubiquitous on the nightside of Venus and can be discerned at tangent altitudes from 80 km to 110 km. The average emission vertical profiles of the c(0)–X(v″) progression and the O₂a(0)–X(0) band, the latter from simultaneous near-infrared spectra, are quite similar, with their respective peaks occurring within ±1 km of each other. We conclude that the net yield for production of the c(0) state is low, ∼1%–2% of the oxygen recombination rate, and that O(³P) and CO₂ are the two likely quenchers of the Herzberg II nightglow, although CO cannot be ruled out. We also derive a value of 2.45 × 10⁻¹⁶ cm³ s⁻¹ for the rate constant at which CO₂ collisionally quenches the c(0) state. Our VIRTIS spectra show hints of O₂A′(0)–a(v″) emission but no traces of the O (¹S–¹D) green line at 557.7 nm.