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

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

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.]