Planetary Aeronomy with Large Telescopes

Much can be done with ground-based telescopes to explore planetary atmospheres, including our own. Over the last four years, we have been using the Keck telescope on Mauna Kea, both in a direct and an indirect manner, to make observations of the terrestrial nightglow and the nightglow of Venus. The spectrographs on the Keck I and II telescopes are very fast echelle systems, and during normal operations they generate so-called sky spectra. To astronomers these are necessary evils - to be subtracted from their target spectra - but to aeronomers they are the best available nightglow spectra in terms of spectral coverage, simultaneity of recording, and spectral resolution. Co-added ASCII files of the Keck nightglow spectra are available on the Web. The use of large telescopes to view planetary atmospheres is an under-used technique, particularly when it comes to Venus and Mars. Since the Venera 9/10 orbiters circled Venus in 1975 there have been no spectroscopic measurements of the planet in the visible spectral region until 1999. At that time we observed Venus for eight minutes with the Keck I telescope, and discovered the oxygen green line, with a comparable intensity (150 R) to the terrestrial value. In a follow-up measurement at the APO telescope in 2001, relatively strong molecular oxygen emission (Herzberg II) was seen, but no green line. Such variability is not presently explicable, particularly when the molecular emission is not simultaneously extinguished, and points out the importance of viewing Venus from some terrestrial observatory at every apparition. In the case of Mars, ground-based viewing is considerably more difficult, since the dark fraction of the planetary disc is never greater than 15%. Observations with the Hubble telescope are more likely to be successful. We gratefully acknowledge support from the NASA Planetary Astronomy program. >http://www-mpl.sri.com/NVAO/download/Osterbrock.html</a>