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.
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