Radio jolts indicate Venusian bolts.

Lightning often flashes across the sky on Earth, and spacecraft have photographed "superbolts" on Jupiter. Uranus, Saturn and Neptune also appear to harbor the telltale crackle of electrical storms, but for years researchers have debated whether Venus experiences such jolts. Data from the orbiting U.S. Pioneer-Venus spacecraft and a series of Soviet craft that landed on the planet remain suggestive but inconclusive. Now, a series of radio bursts detected by the Galileo craft as it swung around Venus last year appear to confirm the existence of lightning on the planet.

A special antenna, designed to detect radio signals from Jupiter when Galileo reaches the giant planet in 1995, took center stage in the Venusian discovery. The antenna's ability to detect higher frequency radio signals than previous craft enabled Galileo scientists to search for characteristic radio burst associated with lightning.

Lightning also produces lower frequency radio waves. But such signals -- like those from a radio station -- would bounce back from Venus' ionosphere rather than pass through it. Thus, most of the waves would remain trapped inside Venus' lower atmosphere and could not reach Galileo, notes Donald A. Gurnett of the University of Iowa in Iowa City. He and his colleagues, who built the antenna, detail their findings in the Oct. 4 SCIENCE.

During a survey that lasted just under an hour an Feb. 10, 1990, the antenna detected nine bursts as it pointed toward the night side of Venus. Although the intensity of the bursts -- transmitted to Earth last December -- only slightly exceeded noise levels in the instrument, Gurnett's team says lightning seems the most likely cause of the signals. The antenna found no such bursts during hour-long control studies conducted in flight before and after the Venus Flyby, they note.

"The findings do indeed indicate that there is lightning on Venus," says William J. Borucki of NASA's Ames Research Center in Mountain View, Calif. "But that doesn't answer the fundamental question: What causes it?"

Some researchers speculate that if Venus still experiences volcanic acitivity, dust particles produced by the flow of molten rock would rub against each other, taking on electric charge and providing the raw material for lightning. But Borucki asserts that this explanation would require an unrealistically high level of volcanism, sinc ethe Venusian lightning may occur as frequently as 100 times a second. Instead, he favors the idea that sunlight on the planet's day side creates updrafts of dust or other particles, which form the basis for lightning. That scenario, he says, could explain why most attempts to photograph lightning flashes on Venus' night side have failed.