Signs of Lightning on Venus

By Richard A. Kerr
ScienceNOW Daily News
28 November 2007

Given that lightning on Earth isn't shy about attracting attention, it might come as a surprise that the phenomenon has been hard to detect on Venus, especially because spacecraft have visited our sister planet more than 30 times. So scientists seemed almost relieved today to report that the orbiting Venus Express spacecraft has returned "the first definitive evidence of abundant lightning on Venus," according to a team member. Chances for lightning seem dimmer on Saturn's moon Titan, however, where new observations by the Cassini spacecraft have failed to detect the phenomenon.

Lightning can change the chemistry of an atmosphere and give researchers clues to the storm activity of a planet. It's common enough in planets with thick atmospheres. Bolts 100 times more powerful than terrestrial ones crackle on Jupiter, and it has been detected on Saturn, on Uranus, and probably on Neptune. After its 1979 arrival at Venus, the Pioneer Venus Orbiter recorded electrical impulses leaking out of the atmosphere that some researchers took to be a signature of lightning. Later, Galileo swept by and detected noise bursts at much higher frequencies, encouraging more faith in venusian lightning (Science, 27 September 1991, p. 1492). But expectations dimmed with Cassini's failure to detect lightning's distinctive radio static during its two flybys.

With the European Space Agency's Venus Express in the neighborhood, space physicist Christopher Russell of the University of California, Los Angeles, and his colleagues were looking for the magnetic signals that should accompany the electrical signals observed by the Pioneer Venus Orbiter. In the 29 November issue of Nature, they report that Venus Express detected bursts of 100-hertz magnetic radiation lasting 0.25 to 0.5 seconds and occurring at least half as often as lightning on Earth. "They were pretty much as we'd predicted," says Russell. "Lightning is occurring beneath the spacecraft about 25%26#37; of the time."

"That is one possibility," says a more skeptical space physicist, Georg Fischer of the University of Iowa in Iowa City. As was the case with the Pioneer Venus data, he says, these signals might instead be generated by processes involving the charged particles and magnetic fields draped around the planet. For his part, Fischer has struck out in finding lightning on Titan. In a paper published online 28 November in Geophysical Research Letters, he and colleagues report that Cassini has failed to detect lightning's radio emissions during the spacecraft's first 35 flybys of the moon.

At Venus, at least, Japanese scientists are looking to break the impasse the old-fashioned way: by catching any lightning flashes with a camera. Their Venus Climate Orbiter is due to arrive in 2010.