Mysterious Night Glow in the Skies
of Venus Puzzles Scientists
By Kenneth Chang
http://www.nytimes. co


What goes on in the atmosphere of Venus above, below and within its clouds of sulfuric acid continues to puzzle scientists. Every time they take a look, they seem to see something different, with phenomena appearing or disappearing like the smile of the Cheshire Cat.
In their latest looks, they saw no signs of lightning, but did see the faint glow of excited oxygen atoms on the night side of Venus. Spacecraft visiting Venus in the 1970's found the exact opposite: signs of lightning, but no oxygen glow.
That leaves the scientists wondering exactly what is going on.
The contradictory evidence for lightning dates back to 1978 when two Soviet spacecraft, Venera 11 and Venera 12, detected electrical discharges as they descended to the surface of Venus.
Later, NASA's Pioneer Venus 1 spacecraft in orbit around the planet recorded low-frequency radio signals that are, at least on Earth, associated with electrical discharges from lightning strikes.
But this evidence was indirect and inconclusive. Some scientists suggested, for example, that the signals that the Venera landers recorded were merely the discharge of static electricity that had built up on the spacecraft themselves as they descended through the atmosphere.
In 1993, a graduate student at the University of Arizona snapped 300,000 pictures of Venus. Seven showed flashes that appeared to be lightning. But a second series of observations turned up no flashes.
When NASA's Galileo spacecraft swung by Venus in February 1990 en route to Jupiter, it recorded bursts of static similar to what is heard on an AM radio during a thunderstorm.
Now, however, Dr. Donald A. Gurnett, a professor of physics at the University of Iowa, reports that the Cassini probe, which swung by Venus twice, in 1998 and 1999, heard no lightning-induced static at all.
"If lightning exists at Venus, it's either extremely rare or much different than Earth lightning," Dr. Gurnett said. Writing in the current issue of Nature, Dr. Gurnett and his collaborators say the new negative result is more persuasive than the earlier Galileo data. Cassini came within 200 miles of the surface "We were just skimming over the atmosphere, practically," Dr. Gurnett said while Galileo was, at its closest, 10,000 miles from the Venus.
When Cassini passed by Earth, it heard the expected lightning-generated static bursts from more than 50,000 miles away.
Dr. Christopher T. Russell, a professor of geophysics and space physics at the University of California at Los Angeles and a strong proponent of lightning on Venus, said the radio receiver was not properly set on the first pass, and the second pass was over the night side, where fewer storms would be expected.
Differences between Venus and Earth could also mean that lightning acts differently on the planets. Venus has an atmosphere of almost all carbon dioxide and its clouds are more than 30 miles above the surface.
While the question of lightning on Venus has been vigorously debated for two decades, the night glow has received little scientific attention since the Venera mission series.
Different types of molecules emit different colors of light. The Soviet probes spotted the colors that indicated the presence of oxygen molecules pairs of oxygen atoms that have bonded together but not the green color given off by excited, single oxygen atoms.
In November 1999, researchers from S.R.I. International in Menlo Park, Calif. and the Lowell Observatory in Flagstaff, Ariz., pointed the 10-meter Keck telescope on Mauna Kea, Hawaii, toward Venus for eight minutes and saw the distinctive green glow of oxygen atoms.
"It was a total surprise," said Dr. Thomas G. Slanger, a scientist at S.R.I. and lead author of a paper in the current issue of Science. The scientists believe the instruments aboard the Venera spacecraft were working correctly they detected the fainter molecular-oxygen glow.
Dr. Slanger speculates that the current more vigorous activity of the Sun it is now near the height of its 11-year cycle may be energizing the sparse oxygen. Only one in 10 million molecules in the atmosphere of Venus is oxygen. At the 60-mile- high altitude where the glow originates, oxygen is more plentiful, but still rare, about one in a thousand.
There is also no easy explanation for what is causing the oxygen atoms to switch on. Unlike Earth, Venus does not possess a magnetic field, which would swing the high-speed electrons and ions of the solar wind around to the back side of the planet.
Dr. Slanger and scientists at NASA's Jet Propulsion Laboratory hope to synchronize observations next month to see whether the glow of molecular oxygen overlaps that of atomic oxygen. That could show something about the chemical reactions in the atmosphere and what is powering the night lights of Venus.
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