Case for "Ashen Light" Weakens -- Sky & Telescope -- May 2001
OVER THE PAST 350 YEARS TELESCOPIC OBSERVERS HAVE occasionally reported a faint glow on the night side of Venus. Although numerous observers have claimed to see this phenomenon, termed the "ashen light," it is so elusive and subtle that many if not most astronomers - professionals and amateurs alike - doubt its existence. Two recent results make the reality of ashen light even more difficult to accept. In November 1999, Tom G. Slanger (SRI International) and his colleagues probed the upper atmosphere of Venus using the 10meter Keck I telescope and a high-resolution spectrometer. They detected a faint, green luminescence on the planet's night side at 557.7 nanometers (5577 angstroms), emission from the relatively few oxygen atoms present among the overwhelming carbon dioxide majority. In the January 19th issue of Science, Slanger's team explains that ultraviolet sunlight splits CO, into carbon monoxide (CO) and atomic oxygen (0) on the planet's day side. Highspeed winds transport these to the night side, where the oxygen atoms recombine into 0, and emit green light in the process. This emission is very weak, Slanger says, as is the night-side airglow from 0, molecules, which the Keck spectrometer detected as well. The combined airglow from Venus does vary a great deal in brightness, and the 5,000-rayleigh value detected through the spectrometer's 7-by-0.86-arcsecond slit was stronger than previously recorded. At that brightness level it might be discernible to an astronaut orbiting over Venus's night side. But it would prove extremely challenging through a backyard telescope, especially since the night side of Venus can be seen well only when the planet is in twilight or very near the horizon. Another possible cause of ashen light - rapid-fire lightning has also become much less promising. During the 1970s Russian and American space probes detected low-frequency radio emissions near Venus, suggesting that the dense atmosphere bristled with nonstop electrical discharges. Visual observers suggested that rapid lightning strikes under the clouds could blend into a steady glow on the planet's night side. However, the case for lightning was never ironclad (S& T.- June 1992, page 610), and in an article appearing in Nature for January 18th, Donald A. Gurnett (University of Iowa) and seven col- leagues cast further doubt on its existence. Gurnett , s team scrutinized data from the Cassini spacecraft, which passed Venus at close range in April 1998 and June 1999. Had lightning been prevalent, the spacecraft would have detected high-frequency radio noise from the strikes, the same static heard on an AM radio during a thunderstorm. Although Cassini easily detected lightning's radio signature during a later flyby of Earth, nothing similar was heard near Venus. "If lightning exists in the Venusian atmosphere," the team concludes, "it is either extremely rare or very different from terrestrial lightning." - J. K. B.
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