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N.J. Chanover, K.S.J. Anderson (New Mexico State Univ.), T.G. Slanger, P.C. Cosby, D.L. Huestis (SRI International)
Ground-based telescopes of large aperture can provide very useful information about planetary atmospheres, but are not often used for this purpose. Spectroscopic observations at high spatial and spectral resolution are particularly valuable for studies of Mars and Venus. Since the Venera 9/10 orbiters circled Venus in 1975 (Krasnopolsky 1983, Venus Spectroscopy in the 3000-8000 Å Region by Veneras 9 and 10, In Venus, The University of Arizona Press), there had been no spectroscopic measurements of the planet in the visible spectral region until 1999. At that time, Venus was observed for eight minutes with the 10-meter Keck I telescope, and the atomic oxygen (OI) green line was discovered, with an intensity (150 R) comparable to the terrestrial value (Slanger et al. 2001, Science 291, 463). The Keck observations also detected the v' = 0 progression of the O2(c-X) Herzberg II system in the 551 nm region, with a total emission intensity of 5 kR. These emissions had also been observed by the Venera probes.
We have made additional spectroscopic observations of Venus during 2001, using the ARC 3.5-meter telescope at Apache Point Observatory. We found strong molecular oxygen emission, but the green line of atomic oxygen was absent. Such strong variability of the atomic oxygen is difficult to explain, particularly when the molecular emission is not simultaneously extinguished. Variability has previously been observed in another O2 system, the 1.27 micron a-X emission, which was first observed in 1979 (Connes et al. 1979, ApJ 233, L29; Crisp et al. 1996, J. Geophys. Res. 101, 4577). These data highlight the need to view Venus from terrestrial observatories with adequate spatial and spectral resolution at every suitable Venus apparition, and to make coordinated measurements at different critical wavelengths. A coordinated campaign will help us to understand the mechanisms responsible for the variability. In the case of Mars, for which there are no nightglow measurements, ground-based viewing is considerably more difficult since, as viewed from Earth, the dark portion of the planetary disk never exceeds 15% of its apparent diameter. Scattered light from the illuminated disk presents severe contamination problems for terrestrial observers; Hubble Space Telescope observations of Mars are more likely to be successful for nightglow detection.
Support from the NASA Planetary Astronomy program is gratefully acknowledged.