SETI bioastro: The Ashen Light of Venus and its implications for exoplanets

From: Larry Klaes (
Date: Tue Apr 25 2000 - 10:54:12 PDT

Ashen light of Venus studied at Bay site

                 By Keay Davidson
                Friday, April 21, 2000

                 They're the ghosts of the solar system -- eerie glows
                 that haunt the night sky and the otherwise dark void
                 between planets.

                 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,
                 emitting light.

                 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
                 atmospheric oxygen.

                 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,"
                 Slanger said.

                 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
                 earthly atmosphere.

                 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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