- 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
- 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
- The contradictory evidence for lightning dates back to
1978 when two Soviet spacecraft, Venera 11 and Venera 12, detected
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
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
- 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
- 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
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
- "If lightning exists at Venus, it's either extremely
rare or much different than Earth lightning," Dr. Gurnett said.
in the current issue of Nature, Dr. Gurnett and his collaborators say the
new negative result is more persuasive than the earlier Galileo data.
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
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
all carbon dioxide and its clouds are more than 30 miles above the
- While the question of lightning on Venus has been
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.,
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
- 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
- There is also no easy explanation for what is causing
the oxygen atoms to switch on. Unlike Earth, Venus does not possess a
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
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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