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University of Iowa News Release

Aug. 6, 2004

Image: This artist concept shows how Cassini is able to detect radio signals from lightning on Saturn. Lightning strokes emit electromagnetic energy across a broad range of wavelengths, including the visual wavelengths we see and long radio wavelengths that cause static on an AM radio during a thunderstorm. Some of the radio waves propagate upwards and can be detected at long distances by the radio and plasma wave science instrument on Cassini.Courtesy NASA. Click here for a high-resolution version of the image.

Cassini Reveals Changes In Thunderstorms At Saturn

University of Iowa researchers and their colleagues have detected lightning in the atmosphere of Saturn using the Radio and Plasma Wave Science (RPWS) instrument carried aboard the Cassini spacecraft. Patterns of occurrence of radio emissions from the lightning suggest differences in thunderstorms from those observed more than 20 years ago.

"We're detecting the same crackle and pop you get on many AM radio broadcasts during a thunderstorm," says University of Iowa research scientist William Kurth, who adds that radio emissions are the only way lightning has been detected at Saturn, given the planet's thick cloud layers. The radio emissions are "bursty," lasting only a tenth of a second or less but are detected at frequencies as low as a few megahertz and higher.

Voyagers 1 and 2 observed radio bursts from lightning during their fly-bys of Saturn in 1980 and 1981, respectively. These earlier observations were of highly regular clusters of lightning occurring approximately every 10 hours, 5 minutes, similar to the rotation period of super-rotating clouds in the equatorial belt of Saturn's atmosphere. Saturn has strong wind bands that encircle the planet and the winds at the equator are faster than those at higher latitudes and faster than the rotation of the planet itself, hence they are termed "super-rotating." Because both Voyagers observed a similar pattern, the storm system apparently lasted for months.

"We expected to see the same behavior with Cassini, but with Cassini, some days you see lightning and some days you don't," he says. "Also, the time between clusters of radio emissions is closer to 10 hours, 45 minutes. This implies that the radio bursts are coming from higher latitudes where the winds are slower. We think that there may be multiple storm systems."

RPWS team member Michael Kaiser of Goddard Space Flight Center, Greenbelt, Maryland, suggests that the differences in storm characteristics seen between Voyager and Cassini may be related to very different ring shadowing conditions. When the Voyager data were recorded, the sun was nearly in the plane containing the rings, which cast a very deep shadow near Saturn's equator. As a result, the atmosphere in a narrow band was permanently in shadow -- making it cold -- and located right next to the area that receives the most direct sunlight in Saturn's atmosphere. This may have caused considerable turbulence and upwelling, leading to long-lived storms. However, during Cassini's approach and entry into Saturn's orbit, it is summer in the southern hemisphere and a more diffuse ring shadow is distributed widely over a large portion of the northern hemisphere.

"We made our first detection at Saturn with Cassini back in 2003 and we were excited because we hadn't expected to detect lightning from so far away," Kurth says. "But then we heard almost nothing until about July 1 when Cassini was placed into orbit around Saturn and we began hearing it more regularly."

Don Gurnett, UI space physicist and Cassini principal investigator, says that lightning can sometimes be detected at great distance from Saturn precisely because Saturn's lightning bolts -- called "superbolts" -- are "staggeringly large" compared to Earth-based lightning.

Kurth's colleagues, in addition to Gurnett and Kaiser, are Philippe Zarka and Alain Lecacheux of the Observatory of Paris, Meudon, France; and Michael Desch of Goddard Space Flight Center, Greenbelt, Maryland.

The radio sounds of Saturn and other sounds of space can be heard by visiting Gurnett's Web site at: http://www-pw.physics.uiowa.edu/space-audio. More information about the Cassini Radio and Plasma Wave Science investigation can be found at http://www-pw.physics.uiowa.edu/cassini/

Cassini, carrying 12 scientific instruments, is the first spacecraft to orbit Saturn and has begun a four-year study of the planet, its rings and its 31 known moons. The spacecraft is part of the Cassini-Huygens Mission that includes the Huygens probe, a six-instrument European Space Agency probe, scheduled to descend through the atmosphere of Titan, Saturn's largest moon, in January 2005.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology, Pasadena, Calif. manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. For the latest images and information about the Cassini-Huygens mission, visit: http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

STORY SOURCE: University of Iowa News Services, 300 Plaza Centre One, Suite 371, Iowa City, Iowa 52242-2500.

CONTACTS: Gary Galluzzo, 319-384-0009, gary-galluzzo@uiowa.edu.