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T.G. Slanger, D.L. Huestis, P.C. Cosby (SRI), T.A. Bida (Keck)
On the morning of November 20, 1999, the night airglow of Venus was observed in the visible spectral region, using the Keck I telescope and the HIRES echelle spectrometer. We report detection of the O(1S-1D) 5577 {Å} green line. The relative velocity of earth and Venus was 12.8 km/s, corresponding to a Doppler shift of 0.24 Å. With a spectral resolution of 0.15 {Å} the terrestrial and Venus green lines were cleanly separated. Scanning from the interior of the disc across the limb, the Venus green line intensity was seen to double before decreasing abruptly beyond the limb, while the terrestrial green line remained constant. After appropriate calibration, we find a Venus 5577 {Å} zenith intensity of 150 R. The 6300 {Å} oxygen red line intensity was immeasurably small, with an upper limit of 20 R. Previous measurements from the Russian Venera 9/10 probes had found a green line upper limit of 5-10 R. The Venera measurements had shown the O2(c1\Sigmau+ - X3\Sigmag-) Herzberg II system to be the dominant emission feature in the visible spectral region. We observe rotational lines of the O2(c - X) 0-10 band surrounding the green line, and assign a total intensity to the O2 system of about 5 kR, consistent with the Venera measurements.
Although the green line is well known in the Earth's atmosphere, the process responsible for production of O(1S) on Venus is unknown. Observation of green line emission in the Venus nightglow demonstrates that it is not necesarily associated with the presence of molecular oxygen in planetary atmospheres.
We gratefully acknowledge support for this study from the NASA Planetary Astronomy program. The W. M. Keck Observatory is operated jointly by the California Institute of Technology and the University of California.
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The author(s) of this abstract have provided an email address for comments about the abstract: tom.slanger@sri.com