SRI International Makes First Observation of Atomic Oxygen Emission in the Night Airglow of Venus

Press Release From: Southwest Research Institute
Posted: Thursday, January 18, 2001

SRI International, a leading research institute based in Silicon Valley, reported the first observation of visible light emitted by oxygen atoms in the night-side airglow (``nightglow'') of Venus. Published in the January 19, 2001 issue of the journal Science, this novel study of a non-Earth atmosphere provides new insight into the atmosphere of Venus and the composition and chemical interactions taking place in the absence of sunlight.

Although much is known about the earth's nightglow, in the visible spectral region there is very limited information concerning Venus or Mars. With the recent discoveries of planets circling other suns, the time may come when it will be possible to view the emissions from these planets and compare them to what is known about our own solar system.

Earth-based Look at Venus is a Hundred-fold Improvement Over 1975's Orbiters

The W. M. Keck telescope and the associated HIRES spectrometer on Mauna Kea, Hawai'i were used by SRI researchers in the Molecular Physics Laboratory to obtain unprecedented detail of the terrestrial nightglow. These results prompted Drs. Tom G. Slanger, David L. Huestis, Philip C. Cosby, and collaborator Thomas A. Bida (currently at Lowell Observatory) to pursue the investigation of the visible nightglow on the dark side of Venus. The only previous measurement was done by the Russian Venera 9/10 orbiters in 1975, with a spectral resolution 100 times inferior to that of Keck-HIRES.

One of the most prominent features in the terrestrial nightglow is the 5577-angstrom atomic oxygen green line, first detected by A. J. Angstrom in 1868, then quantified by Lord Rayleigh in 1930. The Venera probes found this emission to be absent in the Venus nightglow. The apparent difference between the terrestrial and Venusian visible nightglows has been attributed to the different atmospheric compositions: oxygen and nitrogen for the earth, carbon dioxide and nitrogen for Venus.

Initial Discoveries May Inspire Future Extra-Terrestrial Atmospheric Studies

Measurements to record the nightglow of Venus were carried out with the Keck telescope just before sunrise on November 20, 1999. Analysis of the resultant spectrum at the position of the oxygen green line showed strong emission from the terrestrial atmosphere and a comparable signal from Venus, with an intensity some 25 times greater than the upper limits set by the Venera results.

Further measurements will be needed to determine if the Venus atmosphere really exhibits such large variations in green line intensity. Until the present result, the claim could be made that the nightglow green line is only associated with planets with molecular oxygen in their atmospheres. Now there is evidence that this is not the case. Therefore the green line is not an effective diagnostic for atmospheric molecular oxygen, but only indicates the presence of oxygen atoms.

Mars is a potential target for similar observations, which will help to develop a more coherent picture than presently exists of the nightglows of all three planets.

The research was funded by the Planetary Astronomy section of NASA.

About SRI International

Menlo Park-based SRI International ( is one of the world's foremost independent research and technology development organizations. Founded in 1946 as Stanford Research Institute, SRI has been meeting the strategic needs of global markets for more than 50 years. As part of its strategy to bring its high-value innovations to the marketplace, SRI licenses its technologies, forms strategic partnerships, and creates spin-off companies.

SRI's Molecular Physics Laboratory is known worldwide for its fundamental and applied research in collisional and radiative processes involving ions, atoms, and molecules. The group helps clients resolve key problems in areas ranging from lasers to planetary atmospheres and plasmas.

Related Links

  • 19 January 2001: Discovery of the Atomic Oxygen Green Line in the Venus Night Airglow, Science, [summary - can be viewed for free once registered. A subscription fee is required for full access.]

  • 19 January 2001: The Nightside of Venus, Science, [summary - can be viewed for free once registered. A subscription fee is required for full access.]

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