Scientists find giant lightning storm at Saturn
Tue Feb 14, 2006 at 20:40 UTC
Imagine an electrical storm larger than the continental United States in which the lightning bolts are more than 1,000 times stronger than conventional lightning, and you'll have a good idea of the lightning storm -- the strongest of its kind ever seen -- that University of Iowa space scientists and their colleagues currently are tracking at Saturn with the Cassini spacecraft.
UI Professor Donald Gurnett, principal investigator for the Radio and Plasma Wave Science investigation (RPWS), along with UI researchers William Kurth and Georg Fischer, have been tracking the storm since Jan. 23.
"It is clear that this is the strongest lightning activity that we've seen yet with Cassini since it has arrived at Saturn. In fact, the flash rate even exceeds the rate observed by Voyager 1 back in 1980 and the intensities are at least as large, if not larger," Gurnett says. "Since Cassini was over the night side of Saturn and in a difficult position to image clouds, amateur astronomers were asked if they had seen evidence of a storm cloud recently."
He adds that within hours, two amateurs near Paris had posted a beautiful image of a white cloud at southern latitudes on Saturn that they had obtained early on Jan. 25, at a location consistent with the source of the lightning radio emissions being observed by Cassini. Cassini has now imaged the storm that RPWS and the Earth-based amateurs have seen.
Kurth notes that the Iowa-built RPWS instrument detects radio emissions the same way that a car radio picks up the crackle and pop of a summer thunderstorm on Earth.
Image Credit: NASA/JPL/Space Science Institute
This image shows a rare and powerful storm on the night side of Saturn.
Light from Saturn's rings (called "ringshine") provided the illumination, allowing the storm and other cloud features to be seen.
The storm is a possible source of radio emissions believed to come from electrical discharges (lightning) deep in Saturn's atmosphere. Cassini began detecting the radio emissions, which are like those from lightning, on January 23. At about the same time, amateur astronomers reported that a storm had appeared in Saturn's southern hemisphere at minus 35 degrees latitude. Cassini was in the wrong place to take good images of the storm on the day side, since the planet showed only a thin crescent to the spacecraft, but night side imaging was possible using light from the rings.
The image shows the storm as it appeared to the Cassini imaging system on January 27, 2006. The storm's north-south dimension is about 3,500 kilometers (2,175 miles); it is located at minus 36 degrees (planetocentric) latitude and 168 degrees west longitude. This places it on the side of the planet that faces the spacecraft when the radio emissions are detected; the radio emissions shut down for half a Saturnian day when the storm is on the other side.
No lightning flashes are visible in the image. They would look like medium-sized bright spots, since the light would spread out before it reaches the cloud tops. Non-detection does not mean that the lightning is absent, however. Lightning might be too faint to stand out above background or too deep to be seen through the thick clouds. Bad luck is another possibility: The camera might have missed the strong flashes during the 10 seconds that the shutter was open.
A narrow-cloud band crosses the storm from left to right. It is illuminated by the rings from the north and is brighter on that side. Cassini scientists are looking forward to an extensive night side image set, designed to look for lightning. That set will be collected during the first half of this year.
The view was obtained in visible light with the Cassini spacecraft narrow-angle camera at a distance of approximately 3.5 million kilometers (2.2 million miles) from Saturn. The image scale is 20 kilometers (12 miles) per pixel.
Image Credit: NASA/JPL/Space Science Institute
This view was derived from an original Cassini image as seen above by reprojecting it as a cylindrical map and enhancing the contrast to bring out faint features.
"With Cassini we have learned that lightning storms can emerge suddenly and last for several weeks or even a month", says Fischer, a UI postdoctoral research scholar. "On the other hand, we have only observed a single smaller lightning storm throughout 2005, which is remarkably different compared to what we know about terrestrial thunderstorms."
RPWS team member and UI alumnus Michael Kaiser of NASA's Goddard Space Flight Center, Greenbelt, Md., suggests that the storm has varied in intensity, but continued with some 25 episodes occurring since he first noticed the storm on Jan. 23.
The researchers say that the origin of such storms is unknown, but may be related to Saturn's warm interior. Gurnett says that scientists hope to locate the storm with greater precision in the coming weeks when Cassini is scheduled to fly closer to the planet.
Image Credit: NASA/JPL/University of Iowa
Sounds made from radio emissions during a recent lightning storm on Saturn are contained in a podcast at:
http://www.jpl.nasa.gov/multimedia/podcast/saturn-lightning/
The audio sounds like the crackling we hear when we're listening to AM radio during a thunderstorm on Earth. The lightning was captured by the Cassini spacecraft. The podcast also includes an interview with a Cassini scientist, Dr. Bill Kurth of the University of Iowa.
Gurnett's RPWS team colleagues, in addition to Fischer, Kurth, and Kaiser, are Philippe Zarka and Alain Lecacheux of the Observatory of Paris, Meudon, France; and Bill Farrell of Goddard Space Flight Center, Greenbelt, Md.
The Cassini mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif., manages the Cassini mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL.
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