A search for lighting on Venus in 1998 and 1999 using NASA’s
Cassini spacecraft failed to detect high-frequency radio waves
commonly associated with lighting, says physicist Donald Gurnett
of the University of Iowa, Iowa City.

The possible existence of lightning at Venus has long been
controversial. Gurnett and seven co-authors report their negative
findings in this week’s issue of the journal Nature.

“If lightning exists in the Venusian atmosphere, it is either
extremely rare, or very different from terrestrial lightning,”
Gurnett says. “If terrestrial-like lightning were occurring in
the atmosphere of Venus within the region viewed by Cassini, it
would have been easily detectable.”

The Cassini spacecraft, built, operated and managed by
NASA’s Jet Propulsion Laboratory, Pasadena, Calif., made its
closest approach to Jupiter on Dec. 30 and is scheduled to arrive
at Saturn in July 2004. Cassini made two gravity-assisted flybys
of Venus, the first on April 26, 1998, and the second on June 24,
1999. During the flybys the radio and plasma wave science instrument,
with its three, 30-foot-long antennas, searched for impulsive
high-frequency (0.125 to 16 megahertz) radio signals.

Gurnett, who is the principal investigator on the instrument,
says that these signals, called “spherics,” are always produced
by lightning on Earth and are commonly heard as static on AM
radios during thunderstorms. As a test of the instrument’s
ability to detect Earth-generated lightning, a search was
conducted for spherics when Cassini made a close flyby of the
Earth on August 18, 1999. Not surprisingly, the instrument
detected lightning continuously at rates up to 70 impulses per

Despite the Cassini results, Gurnett cannot rule out the
possibility that some type of low-frequency electrical activity
may yet exist at Venus because radio signals cannot penetrate the
ionosphere at frequencies below about 1 megahertz. Therefore, no
definitive statement can be made about the lightning spectrum at
those frequencies

“Since the atmosphere of Venus is very different from that of
Earth, it is perhaps not surprising that electrical activity on
Venus might be very different from lightning in the Earth’s
atmosphere,” says Gurnett, who notes that lightning generally can
be divided into two types, cloud-to-ground and the weaker cloud-
to-cloud variety. “Because clouds over Venus are at very high
altitudes of 40 kilometers (25 miles) or more, it is likely that
lightning at Venus, if it exists, is primarily cloud-to-cloud.
Terrestrial cloud-to-ground lightning is generally more intense
than cloud-to-cloud so it is possible that the absence of
impulsive high-frequency radio signals during the Venus flybys
could be owing to the dominance of very weak cloud-to-cloud
lightning at Venus.”

Gurnett says that electrical activity at Venus could also be
cloud-to-ionosphere discharges. “At the Earth, there is a type of
electrical discharge called a “sprite” that travels up from a
cloud to the ionosphere. A sprite is not like lightning as we
usually think of it,” Gurnett says. “Sprites have a slow
electrical discharge, meaning that they also have a low frequency
and are very difficult to detect.”

Serious discussions over whether lightning exists at Venus
began in 1978 when Venera, Russia’s Venus lander, found low-
frequency signals that some scientists called lightning, but
others doubted for a variety of reasons. Later, physicist William
Taylor, a former University of Iowa student of Gurnett’s, in 1979
found what he considered to be evidence for lightning using the
NASA Pioneer Venus spacecraft. In 1990, using a Galileo spacecraft
instrument similar to the one he designed for Cassini, Gurnett
detected several small impulses that were interpreted at the time
as being indicative of lightning. However, Galileo was some 60
times more distant from Venus than was Cassini, making the
results much less significant than those of Cassini.

Meanwhile, the Cassini spacecraft, launched in 1997, is
continuing its journey to Saturn, where it is scheduled to begin
a four-year exploration of Saturn, its rings, atmosphere and
moons on July 1, 2004. Gurnett and an international team of
some18 co-investigators will use the radio and plasma wave
science instrument to measure Saturn’s powerful radio emissions,
as well as its lightning discharges.

Cassini is a cooperative project of NASA, the European Space
Agency and the Italian Space Agency. JPL, a division of the
California Institute of Technology, in Pasadena, manages Cassini
for NASA’s Office of Space Science, Washington, D.C. Further
information about the mission is available at
http://www.jpl.nasa.gov/cassini .