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 O2 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 O2+ 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(1S) by CO2. An ionospheric source could be interpreted in terms of ion densities [Pätzold et al., 2007]. Although the O(1D) yield is much larger than that of O(1S) from O2+ DR, O(1D) quenching by CO2 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]