Ashen light of Venus studied at Bay site
By Keay Davidson
EXAMINER SCIENCE WRITER
Friday, April 21, 2000
They're the ghosts of the solar system -- eerie glows
that haunt the night sky and the otherwise dark void
Most astronomers are annoyed by these faintly
luminous clouds of gas or dust, which have exotic
names such as "zodiacal light" and "gegenschein."
Their glow is a mildly irritating distraction during
observations of more distant, spectacular sights, such
as super-clusters of galaxies.
But now, scientists at SRI International in Menlo
Park have received a $250,000 NASA grant to
scrutinize one type of celestial specter -- the
legendary "ashen light" of Venus, the second planet
from the sun. The scientists are Tom Slanger and
David Huestis, both chemists who specialize in
aeronomy, the study of the upper atmosphere.
The ashen light is an elusive luminescence,
sometimes faintly visible on the dark side of Venus.
It has enchanted and mystified generations of
stargazers, a few of whom have speculated about its
Perhaps the strangest speculation came from a
German astronomer in the early 19th century. He
suggested that the glow revealed massive political
rallies on the planet Venus, as mobs of Venusians
with torches gathered to honor a new monarch.
A less exotic explanation is preferred by today's
scientists. They believe the ashen light -- technically
known as Venusian nightglow -- comes from oxygen
molecules high in the planetary atmosphere.
On the sunlit side of Venus, Huestis says, solar
ultraviolet radiation bakes atmospheric carbon
dioxide, CO el,4 " el,-4. Each carbon dioxide
molecule consists of one atom of carbon and two of
oxygen. The intense UV light splits a CO el,4 " el,-4
molecule into CO (carbon monoxide, like the poison
gas from cars) and single oxygen atoms.
Then high-altitude Venusian winds blow the CO and
O atoms to the far, unlit side of the planet. This
happens quickly, because Venusian winds blow
incredibly fast -- far faster than the planet below
rotates -- thanks to a still-somewhat-mysterious
process known as "superrotation."
On the dark side, the lone oxygen atoms recombine
into molecular oxygen, that is, O el,4 " el,-4 . As they
recombine, they emit light.
Hence the continual recombination of oxygen atoms
into O el,4 " el,-4 molecules creates a continual faint
glow -- a visual chorus of aerial light. Sometimes the
light is barely enough to be seen through earthly
A related process creates the airglow in Earth's
atmosphere, Huestis says. In the upper atmosphere
of Earth, UV light splits O el,4 " el,-4 molecules into
separate oxygen atoms. These then recombine,
Terrestrial airglow is typically much too faint to be
seen with the naked eye, especially in or near cities,
where electric lighting -- "light pollution," as
astronomers call it -- washes out much of the night
Huestis says Earth's airglow is "about one-fourth as
bright as the Milky Way (galaxy)."
A Web site showing space shuttle photos of
terrestrial airglow, as viewed from Earth orbit, is at
The oxygen explanation of the Venusian airglow was
first suggested in the mid-1970s. That was during the
Cold War, when U.S.-Soviet space competition
spurred the Russians to launch numerous robotic
spaceships toward Venus.
In 1975, two Russian space probes orbited Venus
and used spectrometers to analyze its atmospheric
composition, which is mostly carbon dioxide. The
sensors detected the Venusian airglow; further
analysis of the images identified it as glowing
Now, Slanger and Huestis hope to obtain more
refined measurements by using the giant Keck
telescope in Hawaii. It's perched atop an extinct
volcano on the big island of Hawaii.
The Keck is so powerful, with its 33-foot-wide
mirror, that it can see the Venusian atmosphere more
distinctly than did the Russian space probes, which
flew past the planet.
"The spectral resolution of the Russian (observations)
is more than 100 times worse than Keck can do,"
The scientists also plan to use the Keck telescope to
try to detect airglow on the dark side of Mars, the
fourth planet from the sun (Earth is third), starting in
the spring of 2001. The Venusian observations will
begin in January 2001.
Why study the Venusian night glow at all? It offers
no known practical applications.
The research is interesting for its own sake, at least
partly because it elucidates a natural explanation for
an old astronomical mystery.
By observing the motion of glowing air in the
Venusian atmosphere, scientists can better
understand the dynamics of that atmosphere. That
might, in turn, shed light on the dynamics of the
Also, scientists hope to refine observing techniques
that might be used to study the atmospheric
composition of planets that orbit other stars. Some of
those planets might be inhabited by plants or animals,
and it would be fascinating to learn what kind of
atmosphere they breathe.
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