T. G. Slanger
Molecular Physics Laboratory
SRI International
Menlo Park, CA 94025
Email: slanger@mplvax.sri.com

In order to intelligently consider the appearance of our atmosphere when viewed from a distance platform, it is necessary for us to obtain the maximum information possible concerning its locally-viewed characteristics. The aspects that give hints of life forms are particularly important, and the signatures of planetary atmospheres that are not hospitable to life also need to be recognized.

The terrestrial nightglow has been studied for many years, and great strides have been made in the discipline of aeronomy in explaining the chemical and dynamical behaviour of the atmosphere that leads to the observed optical emissions. As in any field, however, an improvement in technology can have a major impact on our understanding, and just such a situation has recently arisen with realization that the great telescopes, and particularly the 10-meter W. M. Keck Telescope on Mauna Kea, generate atmospheric spectra that are by far the best available for those who wish to study the nightglow.

We can now view the nightglow in much greater detail than was previously possible, and many new features have appeared. In the visible/near IR spectral region (400-900 nm), the general view has been that there is a single band in the O₂ Atmospheric Band system, and a number of bands of the OH Meinel system. The remaining features are atomic - the oxygen red and green lines, sodium, geocoronal hydrogen Balmer-alpha, and very faint nitrogen lines.

The new Keck/HIRES spectra show the nightglow painted with a much richer palette. The single O₂ feature, the b¹_ - X³_ 0-1 band at 865 nm, has now expanded to some 30 bands covering the 650-870 nm region. Instead of seeing emission only from the v = 0 level of the b¹_ state, we now see emission from levels as high as v = 15. From the 0-0 band of this system, very intense when viewed from above the atmosphere, but ostensibly blocked by O₂ when viewed from ground level, we observe that the isotopic O₂ lines are not blocked, and can be easily measured.

As the b-X bands provide fully-resolved rotational progressions, we can use observed rotational temperatures to arrive at conclusions about emission altitudes. For example, we find that a significant fraction of the emission from the v = 1 level arises at a much higher altitude than that from all other levels. The vibrational distribution in the b¹_ state is non-monotonic, exhibiting a minimum in emission from v = 8, and maxima at v = 4 and v = 11.

For the strong OH Meinel band emissions, we are able to detect isotopic features, from ¹⁷OH and ¹⁸OH, as well as emission from a higher vibrational level, v = 10, than was previously observed. OH rotational distributions, showing extremely hot non-LTE behavior, previously seen in a fragmentary manner, are now simultaneously measured for all bands.

The Keck/HIRES data are providing the first identification of the D₁ line of potassium. The 0.02 nm resolution used in these studies makes it possible to differentiate this line from nearby stronger lines of OH. From comparisons of potassium and sodium line intensities, useful information will be developed concerning meteoric input and the chemistry of these species in the atmosphere.

In the 400-600 nm region, where except for the green line and the sodium lines there have been very few identified features, we find that the region is extremely rich with O₂ Herzberg and Chamberlain band emissions. We believe that all three Herzberg systems are present, including the Herzberg II system which is so intense in the visible nightglow of Venus. A comparison of this emission system at Terra and Venus provides very interesting clues as to the atmospheric makeup, and what criteria we might use in differentiating between different types of O₂-containing atmospheres.

In fact, the atmosphere of Venus shows more intense O₂ emission, by an order of magnitude, than is seen on Terra. In spite of this fact, there is essentially no ground-state O₂ in the Venus atmosphere - that generated from the emission itself is rapidly consumed chemically. Observed from an extrasolar planet, without additional information one might well assume that Venus had the atmosphere more capable of supporting animal life. Therefore, the presence of O₂ is a necessary but not sufficient condition for the existence of a hospitable environment. If both O₂ and OH are observed, the situation is more promising, since in our atmosphere, the presence of OH is an indicator of both ozone (and by inference, oxygen) and water.

In any case, the Keck/HIRES spectra have provided new insights into the components of the terrestrial nightglow, and future technological leaps will presumably make it possible to measure spectra in the atmospheres of extrasolar planets and give valid interpretations of their nature.

References:

D. E. Osterbrock, J. P. Fulbright, A. R. Martel, M. J. Keane, S. C. Trager, and G. Basri, "Night-Sky High-Resolution Spectral Atlas of OH and O₂ Emission Lines for Echelle Spectrograph Wavelength Calibration," Pub. Astron. Soc. Pacific 108, 277-308 (1996).

T. G. Slanger, D. L. Huestis, D. E. Osterbrock, and J. P. Fulbright, "The Isotopic Oxygen Nightglow as Viewed From Mauna Kea," Science 277, 1485-1488 (1997).

T. G. Slanger and D. E. Osterbrock, "Aeronomy-Astronomy Collaboration Focuses on Nighttime Terrestrial Atmosphere," EOS, Transactions, American Geophysical Union 79, 149-154 (1998).

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T. G. Slanger, P. C. Cosby, and D. E. Osterbrock, "The O₂(b¹S - X³S ) System in the Terrestrial Nightglow; v' = 0-15," J. Chem. Phys.; in preparation (1999).

D. E. Osterbrock, J. P. Fulbright, P. C. Cosby, and T. A. Barlow, "Faint OH(v = 10), 17OH, and 18OH lines in the Spectrum of the Night Airglow," Publ. Astron. Soc. Pac. 110, 1499-1510 (1998).

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J. T. Trauger and J. I. Lunine, "Spectroscopy of Molecular Oxygen in the Atmospheres of Venus and Mars," Icarus 55, 272-281 (1983).