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Release: Jan. 17, 2001
UI space physicist fails to find evidence of lightning on Venus
IOWA CITY, Iowa -- In an article published in the Jan. 18 issue of the journal
Nature, University of Iowa space physicist Donald Gurnett says that a search
for lightning on Venus in 1998 and 1999 using the Cassini spacecraft failed
to detect high-frequency radio waves commonly associated with lightning. Gurnett's
paper is certain to be of interest to other space physicists for whom the
possible existence of lightning at Venus has long been controversial.
"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, which made its closest encounter with Jupiter on
Dec. 30 and is scheduled to arrive at Saturn in July 2004, made two gravity-assisted
fly-bys of Venus, the first on April 26, 1998 and the second on June 24, 1999.
During the fly-bys the Radio and Plasma Wave Science Instrument (RPWS), with
its three, 30-foot-long antennas, searched for impulsive high-frequency (0.125
to 16 MHz) radio signals. Gurnett, who serves as RPWS principal investigator,
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 RPWS ability to detect Earth-generated lightning,
a search was conducted for spherics as Cassini made a close fly-by of the
Earth on August 18, 1999. Not surprisingly, the instrument detected lightning
continuously at rates up to 70 impulses per second while Cassini was located
closer than 14 Earth radii.
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
MHz. Therefore, no definitive statement can be made about the lightning spectrum
at frequencies below about 1 MHz.
"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 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
fly-bys could be owing to the dominance of very weak cloud-to-cloud lightning
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 UI 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. Under the terms of a $9.6
million NASA contract, Gurnett and an international team of some18 co-investigators
will use the RPWS to measure Saturn's powerful radio emissions, as well as
its lightning discharges.
Gurnett, a member of the National Academy of Sciences, is a veteran of more
than 25 major spacecraft projects, including the Voyager 1 and Voyager 2 flights
to the outer planets, the Galileo mission to Jupiter, and the Cassini mission
to Saturn. He made the first observations of plasma waves and low-frequency
radio emissions in the magnetospheres of Jupiter, Saturn, Uranus and Neptune
and discovered lightning in the atmospheres of Jupiter and Neptune. Gurnett's
University of Iowa co-authors in the Nature article include William Kurth,
George Hospodarsky, and Terry Averkamp.