Flash: NASA's Cassini Spacecraft Sees Lightning On Saturn

WASHINGTON -- NASA's Cassini spacecraft has captured images of
lightning on Saturn. The images have allowed scientists to create the
first movie showing lightning flashing on another planet.

After waiting years for Saturn to dim enough for the spacecraft's
cameras to detect bursts of light, scientists were able to create the
movie, complete with a soundtrack that features the crackle of radio
waves emitted when lightning bolts struck.

"This is the first time we have the visible lightning flash together
with the radio data," said Georg Fischer, a radio and plasma wave
science team associate based at the Space Research Institute in Graz,
Austria. "Now that the radio and visible light data line up, we know
for sure we are seeing powerful lightning storms."

The movie and radio data suggest extremely powerful storms with
lightning that flashes as brightly as the brightest super-bolts on
Earth, according to Andrew Ingersoll, a Cassini imaging science
subsystem team member at the California Institute of Technology in
Pasadena. "What's interesting is that the storms are as powerful --
or even more powerful -- at Saturn as on Earth," said Ingersoll. "But
they occur much less frequently, with usually only one happening on
the planet at any given time, though it can last for months."

The first images of the lightning were captured in August 2009, during
a storm that churned from January to October 2009 and lasted longer
than any other observed lightning storm in the solar system. Results
are described in an article accepted for publication in the journal
Geophysical Research Letters.

To make a video, scientists needed more pictures with brighter
lightning and strong radio signals. Data were collected during a
shorter subsequent storm, which occurred from November through
mid-December 2009. The frames in the video were obtained over 16
minutes on Nov. 30, 2009. The flashes lasted less than one second.
The images show a cloud as long as 1,900 miles across and regions
illuminated by lightning flashes about 190 miles in diameter.
Scientists use the width of the flashes to gauge the depth of the
lightning below the cloud tops.

When lightning strikes on Earth and on Saturn, it emits radio waves at
a frequency that can cause static on an AM radio. The sounds in the
video approximate that static sound, based on Saturn electrostatic
discharge signals detected by Cassini's radio and plasma wave science
instrument.

Cassini, launched in 1997, and NASA's Voyager mission, launched in
1977, previously had captured radio emissions from storms on Saturn.
A belt around the planet where Cassini has detected radio emissions
and bright, convective clouds earned the nickname "storm alley."
Cassini's cameras, however, had been unable to get pictures of
lightning flashing.

Since Cassini's arrival at Saturn in 2004, it has been difficult to
see the lightning because the planet is very bright and reflective.
Sunlight shining off Saturn's enormous rings made even the night side
of Saturn brighter than a full-moon night on Earth. Equinox, the
period around August 2009 when the sun shone directly over the
planet's equator, finally brought the needed darkness. During
equinox, the sun lit the rings edge-on only and left the bulk of the
rings in shadow.

Seeing lightning was another highlight of the equinox period, which
already enabled scientists to see clumps in the rings as high as the
Rocky Mountains.

"The visible-light images tell us a lot about the lightning," said
Ulyana Dyudina, a Cassini imaging team associate based at Caltech,
who was the first to see the flashes. "Now we can begin to measure
how powerful these storms are, where they form in the cloud layer and
how the optical intensity relates to the total energy of the
thunderstorms."

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. NASA's Jet
Propulsion Laboratory in Pasadena manages the mission for NASA's
Science Mission Directorate in Washington. The Cassini orbiter and
its two onboard cameras were designed, developed and assembled at
JPL.

For more information about the Cassini-Huygens mission visit

http://www.nasa.gov/cassini

Source: NASA