The 557.7 nm Oxygen Green Line in the Venus Nightglow

Observations of Venus in 1999 from the Keck I telescope in Hawai’i showed that the oxygen green line can be a relatively strong nightglow feature (~150 R), rivaling the intensity of the terrestrial green line [Slanger et al., 2001]. The emission was not seen in two orbital missions - the Venera 9/10 study, in which the O ₂ Herzberg II bands were first observed [Krasnopolsky et al., 1976], and more recently, the Venus Express (VIRTIS) measurements [Garcia-Muñoz et al., 2009]. Repeated ground-based measurements of the green line have found an intensity varying strongly from apparition to apparition [Slanger et al., 2006]; it has so far not reached the emission level seen in November 1999, at close to solar maximum.

We assume that the source of the green line is either O-atom recombination in the mesosphere, or O ₂ ⁺ dissociative recombination (DR) in the ionosphere, the two main terrestrial processes. The 2007-2008 data used in the VIRTIS/VEX study were co-added over many orbits, during a period when ground-based observations indicated a moderate (~50 R) green line intensity.

In this presentation we consider the argument for a mesospheric vs an ionospheric source. A mesospheric source would be strongly modulated by the temperature-dependent quenching of O( ¹ S ) by CO ₂ . An ionospheric source could be interpreted in terms of ion densities [Pätzold et al., 2007]. Although the O( ¹ D ) yield is much larger than that of O( ¹ S ) from O ₂ + DR, O( ¹ D ) quenching by CO ₂ would preclude its observation and indeed, no oxygen red line was seen in 1999 when the green line intensity was at its peak. [Supported by NASA Planetary Astronomy]

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