http://ciclops.lpl.arizona.edu/view.php?id=897

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 SED 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.

An image associated with this release, and information about the
Cassini-Huygens mission, are available at http://ciclops.org,
http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

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.