The 557.7 nm Oxygen Green Line in the Venus Nightglow

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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]

Garcia-Munoz, A., et al., J. Geophys. Res., (submitted, 2009).
Krasnopolsky, V.A. et al., Cosmic Research, 1976.
Patzold, M. et al., Nature, doi:10.1038/nature06239, 2007
Slanger, T.G., et al., Science, 2001.
Slanger, T.G., et al., Icarus, 2006.


Presented at the 41st Annual Meeting of the Division for Planetary Sciences (DPS) of the American Astronomical Society, Fajardo, Puerto Rico.

Presentation Number 63.07.

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