Cassini Pieces Together Long Standing Saturnian Puzzle
MEDIA RELATIONS OFFICE CASSINI IMAGING CENTRAL LABORATORY FOR OPERATIONS (CICLOPS) SPACE SCIENCE INSTITUTE, BOULDER, COLORADO http://ciclops.org
Preston Dyches (720) 974-5823 CICLOPS/Space Science Institute, Boulder, Colo.
For Immediate Release: February 24, 2005
CASSINI PIECES TOGETHER LONG STANDING SATURNIAN PUZZLE
Cassini may have found the first visible evidence for the powerful radio bursts generated by Saturnian lightning, providing the missing piece of a 24-year old puzzle.
Observations made by both the imaging science experiment and the Radio and Plasma Wave experiment (RPWS) on Cassini indicate that the storm activity in Saturn's atmosphere is responsible for the emission of electrostatic discharges that are akin to those produced by terrestrial lightning.
Both Voyager spacecraft observed Saturn Electrostatic Discharges (SED) during their 1980-81 flybys of the planet. Observations at the time placed the SED source in unseen storm activity in the equatorial region of Saturn shadowed by Saturn's rings, but these observations were indirect and the brief flybys offered no opportunities to test the idea.
Cassini has observed sporadic Saturn electrostatic discharges since July, 2003, but it wasn't until the imaging experiment could resolve storm clouds in the southern hemisphere of the planet that it became possible to correlate these emissions with anything unusual in the appearance of the planet. Saturn's lightning discharges far exceed those produced in terrestrial storms.
Dr. Michael Desch of NASA's Goddard Space Flight Center in Greenbelt, Md. and RPWS Co-Investigator said, "The SED are powerful, impulsive radio bursts that tend to be organized in episodes lasting several hours. Individual episodes sometimes contain hundreds of bursts, and the episodes will often reoccur every 10 to 11 hours - the length of a day on Saturn. But for reasons not at all understood episodes will disappear for long periods of time before starting up again."
Dr. Carolyn Porco, Imaging Team leader and director of CICLOPS at the Space Science Institute in Boulder, Colo., was notified in early August by Michael Desch that a rather strong SED event had taken place in mid-July. Desch wondered if anything unusual had appeared in the images. "There was a bigger than average white storm in the southern hemisphere at that time in a latitude range the imaging team has come to call `storm alley'," she said. "We caught it in the wide angle camera, but we didn't have enough visible observations of the storm to be confident of the correlation with the SEDs. The timing didn't seem right."
When that storm brightened up again in September, imaging team members immediately notified the RPWS team.
The RPWS instrument was in the middle of recording another big outburst of SEDs that made the July-August event seem small. Imaging and RPWS team scientists, including Porco, Dr. Andrew Ingersoll and his associate Dr. Ulyana Dyudina at Caltech, and Desch, combined their observations taken over the course of nearly three weeks. This time a clear pattern emerged.
"We've all now worked out the timing of the two phenomena," said Ingersoll, "and while it is still puzzling, the facts that the largest storm was brightest during major SED events, and that the correlation hangs together over 18 days with the storm always in the same position during an SED episode says they are one and the same phenomena."
This positioning, though consistent, is still strange. The simplest expectation is that the SEDs should peak when the storm is at the center of the Saturn disc, right under the spacecraft, but that is not what is observed. Desch said, "The SED emissions start when the visible storm is below the horizon as viewed from the spacecraft. They stop when the storm is near the disc center and is moving into the sunlit hemisphere. Possibly the visible cloud is trailing behind the lightning source, and the heavy dayside ionosphere is blocking the radio signals. Right now, it's still a most interesting puzzle."
Another puzzle of course is the mere intensity of these outbursts. Dr. Don Gurnett, the RPWS principal investigator at the University of Iowa, said, "The radio signals from Saturn lightning are incredibly intense, almost one million times the intensity of terrestrial lightning. An important question we need to answer is why Saturn's lightning is so intense."
Cassini scientists will continue to monitor SEDs and the correlation with storm activity over the next four years to determine the exact nature of the relationship and to uncover the reason why Saturn's lightning is so much more powerful than Earth's.
The Cassini-Huygens 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, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.