July 7, 2011
Photo: The huge storm churning through the atmosphere in Saturn's northern hemisphere overtakes itself as it encircles the planet in this Feb. 25 true-color view from NASA's Cassini spacecraft. Credit: NASA/JPL-Caltech/SSI.
UI researchers track giant thunderstorm in Saturn's northern hemisphere
If you've ever spent a summer afternoon sitting on a porch swing and watching majestic, white thunder clouds billow skyward, then you know something about the sense of wonder two University of Iowa researchers have experienced during the past seven months as they tracked one of the largest lightning storms ever seen at the planet Saturn.
Professor Don Gurnett and Researcher Bill Kurth in the UI College of Liberal Arts and Sciences Department of Physics and Astronomy described their findings in the July 7 issue of the journal Nature.
In brief, the storm that was first detected on Dec. 5, 2010, as the first observed lightning storm in Saturn's northern hemisphere and continues to be monitored by NASA's Cassini spacecraft is a whopper. Instruments show lightning flashes occurring at a rate 10 times more frequent than previous storms, and the storm itself had grown to almost 4,200 miles in latitudinal diameter by the end of the first week and 6,000 miles within two to three weeks.
Currently, the storm encircles Saturn, covering an area eight times the surface area of Earth.
According to Cassini Radio and Plasma Wave Science team colleague and lead author Georg Fischer of the Space Research Institute, Austrian Academy of Sciences in Graz, Austria, the lightning display is the largest observed by spacecraft orbiting or flying past the planet.
"This storm is thrilling because it shows how shifting seasons and solar illumination can dramatically stir up the weather on Saturn," said Fischer. "We have been observing storms on Saturn for almost seven years, so tracking a storm so different from the others has put us at the edge of our seats."
In recent years, storms of about 1,200 miles in size have been observed at latitudes of around 35 degrees south. This new storm, with up to 10 lightning strikes per second, is occurring at a latitude of 35 degrees north and follows the August 2009 onset of northern hemisphere spring when the sun's rays strike that region more directly. Significantly, it takes Saturn 29.5 Earth years to make one revolution around the sun.
"The seasons at Saturn are accentuated by their long duration," said Kurth. "Shadows of the rings can cover almost an entire hemisphere. Despite most of Saturn's heat coming from the planet itself, the planet's inclination to the sun and the sun's heat are still important to the formation of lightning."
Although it is common to detect 50 lightning strikes per second all around the Earth, Saturn can go for a year or longer without any recorded lightning strikes. But when a big storm occurs, it is dramatic, with extremely intense discharges that can be 10,000 times as powerful as lightning at Earth.
Regarding detection of thunderstorms, Gurnett said that Saturn's over-lying haze layer and light from the rings usually make clouds hard to see, but thunderstorms on Saturn lead to bright white spots because cloud particles from below are transported upward.
Kurth said, "We started getting ground-based support from amateur astronomers. We had used our radio data to inform the Cassini imaging team -- as well as amateur astronomers -- of the location of the lightning."
While the cloud formation is firmly linked to the lightning, the exact way in which the lightning develops remains unclear.
"On Earth, lightning is associated with water and ice where the temperature of the clouds is slightly below freezing," Gurnett said. "At Saturn -- and Jupiter as well -- we're not certain that water is crucial to lightning formation. Saturn has hydrogen and helium in its atmosphere, and ammonia clouds are all over the planet. The current belief is that it's the water, deep in the atmosphere of Saturn, contributing to the clouds."
Kurth said that scientists believe lightning is associated with ice clouds brought to the surface by convective currents. The upward-moving convection currents separate negative and positive electrical charges and make the lightning discharges possible.
The complete Nature paper, "A giant thunderstorm on Saturn," can be found at: http://www.nature.com/nature/journal/v475/n7354/full/nature10205.html.
The research is funded by NASA.
On June 30, 2004, Cassini, carrying 12 scientific instruments, became the first spacecraft to orbit Saturn and began a study of the planet, its rings and its 61 known moons. The $1.4 billion spacecraft is part of the $3.3 billion Cassini-Huygens Mission that includes the European Space Agency's Huygens probe that landed on Saturn's moon Titan 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 Office of Space Science, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. For more, visit: http://www.nasa.gov/mission_pages/cassini/main/index.html.
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